Have default manual URLs redirect to english or browser-requested language if available

[strk]

Urls out there exist that point to:

• ​https://postgis.net/docs/manual-3.4/ST_Contains.html (master branch)
• ​https://postgis.net/docs/manual-3.4/images/st_contains02.png
• ​https://postgis.net/docs/manual-dev/ST_Contains.html (alias of master branch)
• ​https://postgis.net/docs/manual-dev/images/st_contains02.png
• ​https://postgis.net/docs/ST_Contains.html (latest stable branch)
• ​https://postgis.net/docs/images/st_contains02.png

We want them all to continue working BUT the new layout of chunked html (see #5429) imply that a language tag is now required for the html part (not for images) so the above URLs should redirect to (for english version):

• /manual-dev/ -> /manual-3.4 (or whatever master branch will be)
• /manual-3.4/* -> /manual-3.4/$LANG/$1 (only if file does not exist, otherwise it could be an image or a .css file)
• /docs/ -> /manual-3.3 (or whatever latest stable branch is)

[strk]

I've configured temporary redirects (http 302) for the /docs/manual-dev/ urls. Any request for a file ending in .html and not containing dashes is redirected to under the en/ directory.

The request for the root is also redirected, at the moment, still to en/.

Please give it a try.

What is NOT done yet:

• special rule for /manual-3.4 (do we need one or let go ?)
• a plan for when 3.4 becomes stable (the /docs url)