docs: Add slot naming and life cycle ADR

docs/decisions/0003-slot-naming-and-life-cycle.rst

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+====================================
+3. Plugin Slot Naming and Life Cycle
+====================================
+
+Status
+======
+
+Proposed
+
+
+Context
+=======
+
+The Frontend Plugin Framework introduced the concept of plugin slots as a way
+to customize micro-frontends.  Slots are defined in each application's codebase
+with React, currently taking the form:
+
+    <PluginSlot id="arbitrary_slot_name">
+    ...
+    </PluginSlot>
+
+Operators can subsequently insert plugins into this slot by referencing
+"arbitrary_slot_name" in configuration as follows:
+
+    pluginSlots: {
+      arbitrary_slot_name: {
+        plugins: [{
+          op: PLUGIN_OPERATIONS.Insert,
+          widget: {
+            id: 'arbitrary_plugin_name',
+            ...
+          }
+        }]
+      }
+    }
+
+However, the following concerns were identified in relation to completely
+arbitrary slot names:
+
+1. The codebase can become progressively littered with slot names that are
+   unintuitively or inconsistently named, making it harder to document,
+   maintain, and use them
+
+2. There is no expectation that one should be able to infer purpose and
+   location from the slot name
+
+3. While Frontend Plugin Framework supports defining multiple slots with the
+   same name in a frontend app, as the number of slots across the codebase
+   increases it becomes harder and harder for developers to avoid introducing
+   accidental name collisions
+
+4. Without a versioning scheme, there's no way to modify a slot's API without
+   making an implicit breaking change
+
+This is a common problem in computer science, one that has often been addressed
+by use of `reverse domain name notation`_.  It can be seen everywhere, from
+Android package names to Open edX's own specification for `server event
+types`_.
+
+.. _reverse domain name notation: https://en.wikipedia.org/wiki/Reverse_domain_name_notation
+.. _server event types: https://docs.openedx.org/projects/openedx-proposals/en/latest/architectural-decisions/oep-0041-arch-async-server-event-messaging.html#id5
+
+This technique allows for namespace uniqueness within a self-documentated
+hierarchy.  For instance, take this fictitious slot name that uses said
+notation:
+
+    org.openedx.frontend.layout.header.v1
+
+Even without further information, it's possible to tell that:
+
+* The slot belongs to an app in the Open edX org
+* It's a frontend app
+* It's in the app's layout module
+* The slot probably wraps the header
+* This is version 1 of the slot, which indicates changes are possible in the
+  future
+
+And last but not least:
+
+* There's little chance that a slot with the same name exists anywhere in the
+  codebase other than where the layout header is defined
+
+Based on this concept, this ADR aims to define rules that govern how developers
+maintain plugin slots in Open edX frontend apps throughout their lifecycle.  In
+particular, when adding, deprecating, or removing plugin slots.
+
+
+Decisions
+=========
+
+1. Naming format
+----------------
+
+The full name of a plugin slot will be a ``string`` that follows the following
+format:
+
+    {Reverse DNS}.{Subdomain}.{Module}.{Identifier}.{Version}
+
+Where:
+
+* *Reverse DNS* is always ``org.openedx``
+* *Subdomain* is always ``frontend``
+* *Module* denotes the frontend module where the slot is exposed, such as
+  ``courseware``, or ``authoring``
+* *Identifier* is a snake-case string that identifies the slot, which must be
+  unique for the module that contains it
+* *Version* is a monotonically increasing integer prefaced by a `v`, starting
+  with `v1`.
+
+For example:
+
+* org.openedx.frontend.layout.footer.v1
+* org.openedx.frontend.courseware.navigation_sidebar.v2
+
+2. Versioning
+-------------
+
+For the purposes of versioning, a given slot's API contract is comprised of:
+
+* Its location, visual or otherwise, in the Module
+* The type (but not implementation!) of the content it is expected to wrap
+* The specific set of `pluginProps` it exposes
+
+If one of the above changes for a particular slot in such a way that existing
+plugins break or present undefined behavior, *and* if it still make sense to
+use the same Identifier, the version string appended to its name will be
+incremented by `1`.
+
+Note: a given slot's default content is explicitly *not* part of its contract.
+Changes to it do not result in a version bump.
+
+3. Deprecation process
+----------------------
+
+When a slot changes sufficiently to require its version to be incremented, the
+developer will take care to:
+
+* Propose the previous version's deprecation via the official Open edX
+  Deprecation Process
+
+* Keep the definition of the previously released version of the slot in the
+  codebase for the duration of the deprecation process, which should include at
+  least one Open edX release where it co-exists with the new version
+
+* Implement the new version of the slot in such a way that coexists with the
+  previous one with no detriment to either's functionality
+
+
+Consequences
+============
+
+The decisions above are intend to let plugin authors create and maintain
+plugins that are stable across releases of Open edX, while also allowing slots
+themselves to evolve.  The naming convention itself has no significant
+downsides, and while the deprecation process does add some maintenance burden,
+it is expected to be offset by the additional stability provided.