Use Sphinx Inline Tabs to organise installation per OS

[hugovk]

Follow on from #6755 / #6756

Changes proposed in this pull request:

• Use Sphinx Inline Tabs to group the installation instructions by operating system
• This helps hide irrelevant information on long, detailed page
• If, for example, you click the macOS tab in the first tab group, it also opens the macOS tab in the second group
• Because the subheaders are gone, I added in .. _Linux Installation: etc. to avoid linkrot
• I re-ordered them to go: Linux, macOS, Windows, [Windows variant,] FreeBSD, [Android]
  • My thinking was to put the main three first, in order of "likelihood" to needs to follow these instructions, then the others afterwards, but happy to change it

Plus some other cleanup:

• Like our top-level Makefile, inline the .PHONY targets to help avoid omissions in the docs Makefile (and texinfo, info, livehtml and serve were missing)
• Add serve and livehtml to make -C docs help
• EditorConfig: three-space indents for RST

Demo

https://pillow--6764.org.readthedocs.build/en/6764/installation.html