Refactor docserv-stitch: Move checks into XSLT Stylesheets

[tomschr]

Problem

The docserv-stitch command is responsible for additional checks beyond simple structural validation through RNG. At the moment, it is a collection of xmlstarlet commands, which could be better moved into a series of XSLT stylesheets for better readability.

Expected Behaviour

• Refactor docserv-stitch to make it more readable
• Translate xmlstarlet commands into XSLT stylesheets and store them under share/xslt/checks
• Document the purpose of the stylesheets (see check-git.xsl as an example).

[ghost]

• XSLT checks are now supported
• The existing Bash/xmlstarlet checks are now in small individual Bash scripts which should be easier to maintain.
• I reckon some of the existing would not even be implementable in XSLT (or the implementation would be a huge effort).

As such, moving everything into XSLT is a bit of a non-goal for now.