docs: one-click Windows WebDAV mount script + quirks explained

Add scripts/mount-opencoperlock-windows.{ps1,cmd}: configures the
WebClient service (Basic-over-HTTPS, raises the 50 MB cap, timeout),
restarts it, maps the Drive to a chosen letter with a proper label, and
self-elevates only for the registry/service part so the mapping stays
visible in Explorer. Document it in the WebDAV section and explain the two
cosmetic Windows quirks (mount named 'dav', bogus local-disk free space).
Also drop a stray </content> tag left in the doc.

release: support manual workflow_dispatch and self-created tag

Tag pushes aren't possible from every environment, so the Release workflow now also
runs on workflow_dispatch with a tag input; action-gh-release creates the tag from the
branch. Image tags are derived from the resolved version directly (no git-ref dependency).