[Docs] Remove API Reference for downloader, add page about publishing cogs (#3234)

* docs: add info about publishing cogs, remove downloader reference

Co-authored-by: Redjumpman <Redjumpman@users.noreply.github.com>

* chore(changelog): add towncrier entries

* docs: fix broken reference in 3.1.0 changelog

Co-authored-by: Redjumpman <Redjumpman@users.noreply.github.com>

docs/guide_publish_cogs.rst

@@ -0,0 +1,85 @@
+.. Publishing cogs for V3
+
+Publishing cogs for Red V3
+==========================
+
+Users of Red install 3rd-party cogs using Downloader cog. To make your cog available
+to install for others, you will have to create a git repository
+and publish it on git repository hosting (for example `GitHub <https://github.com>`_)
+
+Repository Template
+-------------------
+
+We have standardized what a repository's structure should look like to better assist
+our Downloader system and provide essential information to the Red portal.
+
+The main repository should contain at a minimum:
+
+ - :ref:`An info.json file <info-json-format>`
+ - One folder for each cog package in the repository
+
+   - refer to :doc:`/guide_cog_creation` for information on how to create a valid cog package
+   - you should also put :ref:`info.json file <info-json-format>` inside each cog folder
+
+We also recommend adding a license and README file with general information about the repository.
+
+For a simple example of what this might look like when finished,
+take a look at `our example template <https://github.com/Cog-Creators/Applications>`_.
+
+.. _info-json-format:
+
+Info.json format
+----------------
+
+The optional info.json file may exist inside every package folder in the repo, 
+as well as in the root of the repo. The following sections describe the valid 
+keys within an info file (and maybe how the Downloader cog uses them).
+
+Keys common to both repo and cog info.json (case sensitive)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+- ``author`` (list of strings) - list of names of authors of the cog or repo.
+
+- ``description`` (string) - A long description of the cog or repo. For cogs, this 
+  is displayed when a user executes ``[p]cog info``.
+
+- ``install_msg`` (string) - The message that gets displayed when a cog 
+  is installed or a repo is added
+  
+  .. tip:: You can use the ``[p]`` key in your string to use the prefix
+      used for installing.
+
+- ``short`` (string) - A short description of the cog or repo. For cogs, this info 
+  is displayed when a user executes ``[p]cog list``
+
+Keys specific to the cog info.json (case sensitive)
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+- ``min_bot_version`` (string) - Min version number of Red in the format ``MAJOR.MINOR.MICRO``
+
+- ``max_bot_version`` (string) - Max version number of Red in the format ``MAJOR.MINOR.MICRO``,
+  if ``min_bot_version`` is newer than ``max_bot_version``, ``max_bot_version`` will be ignored
+
+- ``min_python_version`` (list of integers) - Min version number of Python
+  in the format ``[MAJOR, MINOR, PATCH]``
+
+- ``hidden`` (bool) - Determines if a cog is visible in the cog list for a repo.
+
+- ``disabled`` (bool) - Determines if a cog is available for install.
+
+- ``required_cogs`` (map of cogname to repo URL) - A map of required cogs that this cog depends on.
+  Downloader will not deal with this functionality but it may be useful for other cogs.
+
+- ``requirements`` (list of strings) - list of required libraries that are
+  passed to pip on cog install. ``SHARED_LIBRARIES`` do NOT go in this
+  list.
+
+- ``tags`` (list of strings) - A list of strings that are related to the
+  functionality of the cog. Used to aid in searching.
+
+- ``type`` (string) - Optional, defaults to ``COG``. Must be either ``COG`` or
+  ``SHARED_LIBRARY``. If ``SHARED_LIBRARY`` then ``hidden`` will be ``True``.
+
+.. warning::
+    Shared libraries are deprecated since version 3.2 and are marked for removal in version 3.3.
+