fix+enh(sphinx): Remove dup versions in title + weird title combination + don't use `latest` on older docs + add real version

[joshtrichards]

☑️ Resolves #7640

• Fixes #7640
• Helps with #1577

Summary

• Eliminates duplicate version string appearing in HTML title (e.g. Introduction — Nextcloud latest Administration Manual latest documentation
• Replaces latest with actual version string (or branch name if it's an arbitrary branch other than master or stableXX)
• Replaces latest (for master doc builds) with upcoming to match their label on the https://docs.nextcloud.com + clearer indicate it's actually the documentation for the upcoming release of Nextcloud not necessarily the "latest"
• Eliminates long version string that was included in the User Manual's chapter names

Caveat: Unless we backport this to all published older stableXX branches, all of the old documentation will continue to say latest. Once we merge we can try seeing how far back we can backport all this... Either way it'll be better for the most recent 3 releases (and going forward).

TODO:

• ~~Add v before version number~~
• [x] Handle unknown / master
• [ ] Double check short title usage spots
• [ ] Decide whether to drop Server from the titles (like it was previously)
• [x] Figure out why my PDF builds seem to not be using the theme

🖼️ Screenshots

Admin

Before:

html_title:

• If master: Introduction — Nextcloud latest Administration Manual latest documentation
• If stable28: Introduction — Nextcloud latest Administration Manual latest documentation
• If stable27: Introduction — Nextcloud latest Administration Manual latest documentation
• If randomBranch: Introduction — Nextcloud latest Administration Manual latest documentation

pdf:

After:

html_title:

• If master: Introduction — Nextcloud Server (upcoming) Administration Manual
• If stable28: Introduction — Nextcloud Server (28) Administration Manual
• If stable27: Introduction — Nextcloud Server (27) Administration Manual
• If randomBranch: Introduction — Nextcloud Server (randomBranch) Administration Manual`

pdf:

User

Before

html_title:

• If master: Nextcloud latest user manual introduction — Nextcloud latest User Manual latest documentation
• If stable28: Nextcloud latest user manual introduction — Nextcloud latest User Manual latest documentation
• If stable27: Nextcloud latest user manual introduction — Nextcloud latest User Manual latest documentation
• If randomBranch: Nextcloud latest user manual introduction — Nextcloud latest User Manual latest documentation

toc:

After

html_title:

• If master: Introduction — Nextcloud Server (upcoming) User Manual
• If stable28: Introduction — Nextcloud Server (28) User Manual
• If stable27: Introduction — Nextcloud Server (27) User Manual
• If randomBranch: Introduction — Nextcloud Server (randomBranch) User Manual

toc:

PDF title page:

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