Simplified anchors

[radarhere]

Suggestion for python-pillow#7858

.. _Building on Linux:
.. _Building on macOS:
.. _Building on Windows:
.. _Building on Windows using MSYS2/MinGW:
.. _Building on FreeBSD:
.. _Building on Android:

and

.. _Linux Installation:
.. _macOS Installation:
.. _Windows Installation:
.. _FreeBSD Installation:

are anchors that were added in python-pillow@e3a46fcfd0111d7f080da0efe5846 to replace headings.

I think they're not needed in these new locations. If someone was linking to https://pillow.readthedocs.io/en/stable/installation.html#building-on-windows-using-msys2-mingw, it's not going to do any good for us to add that anchor at building-from-source.html, as it's a different page.

[hugovk]

Makes sense, thanks!