[th/generate-doc-fixes] improve generation of manual pages

Merged Thomas Haller requested to merge th/generate-doc-fixes into main

how we generate our manual pages is rather complicate (multiple XMLs are generated and merged).

Try to simplify that (or at least, make it more sensible).

Note that previously overrides like ---nmcli--- for connection.uuid did not end up in man nm-settings-nmcli. That is fixed now.

Also adjust a few documentation texts.

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