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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Note that these are only a first few sections of https://qa.php.net/phpt_details.php, so the format can be determined/discussed before porting the whole documentation (including https://qa.php.net/write-test.php).

cc @derickr

[iluuu1994]

Unfortunately, rstfmt doesn't support custom directives 😞 There were multiple reports about it but I didn't find a workaround. If we require them at some point, I don't object to dropping the tool. It's more important to have good documentation than nicely formatted one.

[cmb69]

Unfortunately, rstfmt doesn't support custom directives

No problem; I've already updated the PR. It might be a good idea to add the rstfmt check to make.bat, and maybe to also provide a make.sh doing the same.

I'm going to also provide a PR for an MD alternative; then we can see what we like better.