Trigger Quirks mode for index/glossary/search HTML pages

Setting with DOCTYPE the public identifier, but not the system identifier
will with 4.01 Transitional trigger the Quirks mode.

To match that also make some tags omit the "/", so the validator is more
happy.

The used kdoctools CSS files (as of KF 6.1) seem to require Quirks mode
for now, otherwise those CSS files will not be loaded and skipped.

Other generated HTML pages (app manuals from kdoctools, man page) also
currently trigger Quirk mode, either by not setting a DOCTYPE or also
omitting the system identifier.

For the magic how to trigger Quirks mode, see also
https://en.wikipedia.org/wiki/Quirks_mode

(cherry picked from commit 744e19e2)