docs for source_credentials.json (#3149)

* docs for source_credentials.json

* Update reference/config_files/source_credentials.rst

Co-authored-by: Rubén Rincón Blanco <git@rinconblanco.es>

* Update reference/config_files/source_credentials.rst

Co-authored-by: Rubén Rincón Blanco <git@rinconblanco.es>

* Update reference/config_files/source_credentials.rst

Co-authored-by: Rubén Rincón Blanco <git@rinconblanco.es>

* Update reference/config_files/source_credentials.rst

Co-authored-by: Rubén Rincón Blanco <git@rinconblanco.es>

---------

Co-authored-by: Rubén Rincón Blanco <git@rinconblanco.es>

reference/config_files.rst

@@ -13,3 +13,4 @@ These are the most important configuration files, used to customize conan.
     config_files/profiles
     config_files/settings
     config_files/remotes
+    config_files/source_credentials

reference/config_files/source_credentials.rst

@@ -0,0 +1,81 @@
+.. _reference_config_files_source_credentials:
+
+source_credentials.json
+=======================
+
+.. include:: ../../common/experimental_warning.inc
+
+
+When a ``conanfile.py`` recipe downloads some sources from other servers with the ``download()`` or the ``get()`` helpers like:
+
+.. code-block:: python
+
+    def source(self):
+        # Immutable source .zip
+        download(self, f"https://server/that/need/credentials/files/tarballname-{self.version}.zip", "downloaded.zip")
+        # Also the ``get()`` function, as it internally calls ``download()``
+
+
+These downloads would be typically anonymous for open-source third party libraries in the internet, but it 
+is also possible that some proprietary code in a private organization or provided by a vendor would require
+some kind of authentication.
+
+For this purpose the ``source_credentials.json`` file can be provided in the Conan cache. This file
+has the following format, in which every ``credentials`` entry should have a ``url`` that defines the
+URL that should match the recipe one. If the recipe URL starts with the given one in the credentials files,
+then the credentials will be injected. If the file provides multiple credentials for multiple URLs, they
+will be evaluated in order until the first match happens. If no match is found, no credentials will be injected.
+
+.. code-block:: json
+
+    {
+        "credentials": [
+            {
+                "url": "https://server/that/need/credentials", 
+                "token": "mytoken"
+            }
+        ]
+    }
+
+Using the ``token`` field, will add an ``Authorization = Bearer {token}`` header. This would be the preferred
+way of authentication, as it is typically more secure than using user/password.
+
+If for some reason HTTP-Basic auth with user/password is necessary it can be provided with the ``user`` and
+``password`` fields:
+
+.. code-block:: json
+  
+    {
+        "credentials": [
+            {
+                "url": "https://server/that/need/credentials", 
+                "user": "myuser", 
+                "password": "mypassword"
+            }
+        ]
+    }
+
+As a general rule, hardcoding secrets like passwords in files is strongly discouraged. To avoid it, the 
+``source_credentials.json`` file is always rendered as a jinja template, so it can do operations like 
+getting environment variables ``os.getenv()``, allowing the secrets to be configured at the system or CI
+level:
+
+.. code-block:: jinja
+    
+    {% set mytk = os.getenv('mytoken') %}
+    {
+        "credentials": [
+            {
+                "url": "https://server/that/need/credentials", 
+                "token": "{{mytk}}"
+            }
+        ]
+    }
+
+.. note::
+
+    **Best practices**
+
+    - Avoid using URLs that encode tokens or user/password authentication in the ``conanfile.py`` recipes. These URLs can easily leak into logs, and 
+      can be more difficult to fix in case of credentials changes (this is also valid for Git repositories URLs and clones,
+      better use other Git auth mechanisms like ssh-keys)