Post: Use build.os instead of build.image (#233)

* Post: Use `build.os` instead of `build.image`

Announcement for readthedocs/readthedocs.org#10589

* Apply suggestions from code review

Co-authored-by: Eric Holscher <[email protected]>

* Update date

---------

Co-authored-by: Eric Holscher <[email protected]>

Author: humitos

use-build-os-config.rst

@@ -0,0 +1,59 @@
+.. post:: August 9, 2023
+   :tags: builders, deprecation
+   :author: Manuel
+   :location: BCN
+   :category: Changelog
+
+
+Use ``build.os`` instead of ``build.image`` on your configuration file
+======================================================================
+
+We are announcing the deprecation of ``build.image`` config key in favor of ``build.os``.
+Read the Docs *will start requiring* a ``build.os`` config key for all projects in order to build documentation successfully.
+**We will start failing builds using "build.image" on their config file on October 16, 2023**.
+
+
+Deprecation timeline
+--------------------
+
+We understand this change will affect many of our users,
+so we have a timeline to communicate this deprecation to our users effectively.
+
+* **Monday, August 28, 2023**: Do the first brownout (temporarily enforce this deprecation) for 12 hours: 00:01 PST to 11:59 PST (noon)
+* **Monday, September 18, 2023**: Do a second brownout (temporarily enforce this deprecation) for 24 hours: 00:01 PST to 23:59 PST (midnight)
+* **Monday, October 2, 2023**: Do a third and final brownout (temporarily enforce this deprecation) for 48 hours: 00:01 PST to October 3, 2023 23:59 PST (midnight)
+* **Monday, October 16, 2023**: Fully remove support for building documentation using "build.image" on the configuration file
+
+
+Migrating to ``build.os``
+-------------------------
+
+If you have a project on Read the Docs that is using ``build.image``,
+**you will need to migrate to the new config key as soon as possible** to continue building your project.
+
+There are some small differences between ``build.image`` and ``build.os`` that we detail here:
+
+- ``version: 2`` must be used with ``build.os``
+- To specify the Python version, you have to use ``build.tools.python``
+
+Below is an example of a valid ``build`` section of your configuration.
+Make sure to make these changes before **October 16, 2023**:
+
+
+.. code:: yaml
+
+   version: 2
+   build:
+     os: "ubuntu-22.04"
+     tools:
+       python: "3.11"
+
+You can read about the ``build.os`` key, including all possible values, in our `"build.os" documentation <https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os>`_.
+
+Contact us
+----------
+
+`Contact us`_ if you have any questions,
+and let us know if you are having trouble using a this new config key for any reason.
+
+.. _Contact us: https://readthedocs.org/support/