Use only one variant of the config file

Why `.readthedocs.yaml`?

- Configuration files usually start with a `.` so they are hidden in
  unix like systems.
- `.yaml` is the official and recommended extension for yaml files.

We could go a little further and use a feature flag to stop finding
the other ones.

ref #7915

Author: stsewd

docs/config-file/index.rst

@@ -3,12 +3,12 @@ Configuration File
 
 In addition to using the admin panel of your project to configure your project,
 you can use a configuration file in the root of your project.
-The configuration file can be named:
+The configuration file should be named ``.readthedocs.yaml``.
 
-- ``readthedocs.yml``
-- ``readthedocs.yaml``
-- ``.readthedocs.yml``
-- ``.readthedocs.yaml``
+.. note::
+
+   Some other variants like ``readthedocs.yaml``, ``.readthedocs.yml``, etc
+   are deprecated.
 
 The main advantages of using a configuration file over the web interface are:
 

docs/config-file/v1.rst

@@ -15,7 +15,7 @@ Here is an example of what this file looks like:
 
 .. code:: yaml
 
-   # .readthedocs.yml
+   # .readthedocs.yaml
 
    build:
      image: latest

docs/config-file/v2.rst

@@ -2,8 +2,8 @@ Configuration File V2
 =====================
 
 Read the Docs supports configuring your documentation builds with a YAML file.
-The :doc:`configuration file <index>` must be in the root directory of your project.
-We recommend the filename ``.readthedocs.yml``.
+The :doc:`configuration file <index>` must be in the root directory of your project
+an be named ``.readthedocs.yaml``.
 
 All options are applied to the version containing this file.
 Below is an example YAML file which shows the most common configuration options:
@@ -14,7 +14,7 @@ Below is an example YAML file which shows the most common configuration options:
 
       .. code:: yaml
 
-         # .readthedocs.yml
+         # .readthedocs.yaml
          # Read the Docs configuration file
          # See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
 
@@ -39,7 +39,7 @@ Below is an example YAML file which shows the most common configuration options:
 
       .. code:: yaml
 
-         # .readthedocs.yml
+         # .readthedocs.yaml
          # Read the Docs configuration file
          # See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
 

docs/intro/import-guide.rst

@@ -72,7 +72,7 @@ and to troubleshoot any issues that arise.
 
 Some documentation projects require additional configuration to build
 such as specifying a certain version of Python or installing additional dependencies.
-You can configure these settings in a ``readthedocs.yml`` file.
+You can configure these settings in a ``.readthedocs.yaml`` file.
 See our :doc:`/config-file/index` docs for more details.
 
 It is also important to note that the default version of Sphinx is ``v1.8.5``.