readthedocs website + pdfs generated from markdown in NSIS

[mcarans]

• Readthedocs website https://oolite.readthedocs.io/en/latest/
• Major simplification of build_all workflow to remove DOxygen generation, pdf generation and website generation
• Readthedocs website hosts pdfs as well as markdown and DOxygen
• OoliteReadMe.pdf is generated from Documentation/docs/index.md and AdviceForNewCommanders.pdf from Documentation/docs/advice.md by the readthedocs build process
• Reference sheet still comes from the last remaining odt in docs as its layout is not suitable for md - the readthedocs process generates the pdf and hosts it.
• pdfs are downloaded from readthedocs website when building nsis
• New pdfs added to nsis - License.pdf and Privacy.pdf
• Make error messages consistent in existing shell scripts
• Move much of GNUmakefile.postamble into a shell script
• Bring Flatpak build in line with FlatHub submission

Once merged I'll edit the readthedocs config in the UI to point to the Oolite repo master branch instead of my fork's readthedocs branch.

[AnotherCommander]

Due to the number of files this PR touches and the quite radical changes it proposes, I think it would be best to merge it after 1.92 so that we won't have to modify a working release pipeline just before release.

[mcarans]

I also moved most of the GNUmakefile.postamble logic into a bash file. I tested building and running on Windows legacy and modern as well as Linux.

[mcarans]

@AnotherCommander I've brought this PR up to date with master changes