docs(adrs): add monorepo adr (#1191)

adrs/0003-monorepo.md

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+# Transition Stacks to a monorepo
+
+**Date:** 2022-11-28
+
+**Related PR:** #1191
+
+## Issue
+
+The Stacks repo is growing in complexity and will soon require many different dependencies that are optional for most consumers.
+
+## Decision
+
+Despite shipping many different concepts (atomic classes, CSS components, JS components), the interdependency chains are relatively linear. This means that we can fairly trivially split the repo into smaller portions.
+
+Our current codebase could be split like so:
+- Atomic classes (no dependencies)
+- CSS Components (depends on Atomic components)
+- JS Components (depends on Stimulus, Popper.js, CSS + Atomic components)
+- Documentation site (depends on JS components, 11ty + plugins)
+
+When the codebase expands in the future, we'll be able to add the following without worrying about consumers needing to import dependencies they don't need:
+- Front-end component library or libraries (e.g. React, Web Components)
+- Large, complex components (e.g. autocomplete)
+- Extra build chain items, such as a [Style Dictionary](https://amzn.github.io/style-dictionary/)
+
+We'd also be able to fold the [Stacks-Icons](https://github.com/StackExchange/Stacks-Icons) and [Stacks-Utils](https://github.com/StackExchange/Stacks-Utils) repos into this main monorepo as well.
+
+## Additional Info
+
+On the technical front, we could manage the monorepo with [npm workspaces](https://docs.npmjs.com/cli/v9/using-npm/workspaces?v=true) or another dedicated tool such as [Rush](https://rushjs.io/pages/intro/welcome/), [Lerna](https://lerna.js.org/) or [Bit](https://bit.dev/).
+
+In regards to changelog / release notes generation, we'd continue using Conventional Commits, but moving to use [release-please](https://github.com/googleapis/release-please) which [does support monorepos](https://github.com/googleapis/release-please/blob/main/docs/manifest-releaser.md).
+
+For backwards compatibility, we can trivially create a `@stackoverflow/stacks` "meta-package" that simply imports, then re-exports the dependency packages.