[mistraloz]

Summary

Depends on #426 (this PR contains its commit, the diff will shrink once it is merged).

This PR is a maintenance update of the documentation:

• Bug
• Feature
• Breaking changes

php2shell is no longer the main news of the images. The README now highlights the fpm variant with its built-in Apache, the recommended way to serve a web application (2 to 3 times more traffic with 4 times less memory than mod_php, see the benchmark):

• the README banner presents the built-in Apache of the fpm variant instead of php2shell;
• the usage examples start with the fpm variant and its built-in Apache, the apache variant comes last;
• the apache section of docs/variants.md recommends the fpm variant for new applications;
• php2shell is mentioned in the supported versions, and its page stays available;
• a new Roadmap section links to the v6 roadmap (Roadmap #359).

It also documents two questions of #190 in docs/fpm.md:

• an application outside /var/www/html (APACHE_DOCUMENT_ROOT and working directory);
• additional PHP-FPM pools with their own settings, routed by the built-in Apache (LocationMatch + SetHandler) or
  by your own web server.

Fixes #190

Test plan (required)

Documentation only: make blueprint, links checked. The examples of docs/fpm.md were run on a PHP 8.4 fpm image:

• /info.php is served by the www pool (memory_limit=128M), /wp-admin/info.php by the admin pool
  (memory_limit=512M), a missing /wp-admin/*.php file gets a 404, the workers of both pools run as docker;
• the application mounted in /var/www with APACHE_DOCUMENT_ROOT=/var/www/public is served;
• without the built-in Apache, the admin pool starts without warning.

Checklist

• I followed the guidelines in CONTRIBUTING guide
• I have squashed any insignificant commits
• This change has comments for package types, values, functions, and non-obvious lines of code