fix: resolve 37 broken links detected by lychee CI checker

[Copilot]

The lychee link checker added in #14231 found 37 broken links because the commit converting absolute https://docs.nextcloud.com/server/latest/… URLs to relative paths used .rst extensions — but lychee checks the built HTML stage, where only .html files exist.

Changes

• .rst → .html in all relative cross-manual hyperlinks across admin, developer, and user manuals
• mysql_4byte_support.rst — reverted self-link back to absolute URL targeting v20 docs (the #mariadb-10-2-or-earlier section no longer exists in current docs)
• coding_standards/index.rst — fixed relative path depth for codereviews link (../ → ../../)
• user_manual/desktop/uninstallation.rst — fixed 4 links pointing to non-existent admin_manual/desktop/configfile.html; the file lives in user_manual/desktop/, so changed to relative configfile.html#configuration-file
• user_manual/files/access_webdav.rst — fixed self-referencing link access_webdav.rst#anchor → #anchor
• webdav.rst / office/configuration.rst — wrapped bare https:// and http:// text in inline code literals ( https:// ) to prevent docutils from auto-linking them as relative broken paths
• .github/workflows/sphinxbuild.yml — added --exclude-path '.*/_static/.*' to lychee config to skip template files (e.g. openapi.html) that reference CDN resources not present in the stage

🖼️ Screenshots

[nickvergessen]

are you sure this is working/a good idea?
As far as I recall the RST links got resolved to HTML for HTML and also worked in the PDF by scrolling to the related section.

Not they are relative links in the PDF that lead to a relative HTML page that does not exist on the server / local PC?

But for cross manual like here we actually need to use absolute links