Move PHPT documentation to docs

[cmb69]

This was so far hosted on qa.php.net, but since there are plans to retire that site, we move the relevant documentation about PHPTs to php-src/docs.

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This is an alternative to PR #15939, but this time using Markdown.

[iluuu1994]

For context on the new documentation:

https://externals.io/message/122357#122361

I switched to rst after running into some formatting difficulties, and some strong opinions on MD on Slack 😄

[cmb69]

See https://externals.io/message/125480#125588 for why I also did this PR.

I'm more comfortable writing MD than RST, but it's not a big deal for me using the latter. Even HTML or DokuWiki markup would be okay for me for the PHPT docs.

[mvorisek]

I can highly recommend myst_parser which allows you to write everything in Markdown, and if something is unsupported by Markdown, it allows you to write that part in inlined full featured RichText.

[cmb69]

Thank @mvorisek! I'd rather not add yet another markup language to my skillset. :)

Anyhow, I'm closing this PR in favor of #15939.