The fork's documentation on the wiki, and the history left alone

Building, upstream tracking and YTSage's own README moved to the wiki; README
points there.

The history is not rewritten, unlike every other repository here. This one
still merges from oop7/YTSage and a rewrite would change every commit id, which
costs more than a clean history is worth in a fork. .github/ and
readme-translations/ are upstream's files and stay where upstream put them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -20,7 +20,7 @@ with the full yt-dlp download feature set inherited from
 **Downloading** (from YTSage)
 - Format table, audio extraction, subtitles, SponsorBlock, chapters,
   playlists, trimming, speed limits, proxies, cookies, custom yt-dlp commands,
-  download history — see the [upstream README](docs/UPSTREAM_README.md)
+  download history — see the [upstream README](https://git.houmeres.sk/Houmeres/SageTube/wiki/Upstream-README)
 
 ## Install
 
@@ -30,14 +30,14 @@ sagetube
 ```
 
 Requires **libmpv** for playback (Arch: `pacman -S mpv`) — see
-[docs/BUILDING.md](docs/BUILDING.md). Without it the app works as a downloader.
+[Building](https://git.houmeres.sk/Houmeres/SageTube/wiki/Building). Without it the app works as a downloader.
 
 ## Fork notes
 
 SageTube is a fork of [YTSage](https://github.com/oop7/YTSage) by
 [oop7](https://github.com/oop7) (MIT). The internal package keeps the
 `ytsage` name so upstream fixes remain mergeable — see
-[docs/UPSTREAM.md](docs/UPSTREAM.md). The fork also fixes a number of
+[Tracking upstream](https://git.houmeres.sk/Houmeres/SageTube/wiki/Tracking-upstream). The fork also fixes a number of
 upstream bugs (unsafe partial-file cleanup, unverified binary updates,
 thread-safety issues, PATH corruption on Windows — see the git log).