docs: use the maintained main-nightly docker tags (#550)

Fixes https://github.com/go-gitea/gitea/issues/39310

The docker install pages tell readers to use `:nightly` / `:nightless-rootless` for the latest development version, but those tags are no longer published — the registry only carries `main-nightly` and `main-nightly-rootless` (plus the per-release-branch `1.x-nightly` tags):

```
$ curl -s https://docker.gitea.com/v2/gitea/tags/list?n=1000 | tr ',' '\n' | grep -E '"(main-)?nightly(-rootless)?"'
"main-nightly"
"main-nightly-rootless"
```

Anyone who followed the rootless page has been pinned to an image that has not been rebuilt since 2025-08-23.

Changes:

- point the docker and rootless docker install pages at `:main-nightly` / `:main-nightly-rootless`, in English and in the zh-cn/zh-tw translations
- use `1.27-nightly` instead of the long gone `1.16-nightly` as the release branch example (the oldest one still published is `1.19-nightly`)
- fix the zh-cn/zh-tw claim that `:latest` installs a development version, and mention the nightly tags there the way the English page does
- render `@dockerVersion@` as `main-nightly` for the next version, so the compose snippets on the unreleased docs no longer reference `gitea:nightly`

Not touched: the 1.22–1.26 versioned docs carry the same stale tag, but those versions are already out of maintenance — happy to include them if preferred.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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---------

Co-authored-by: bircni <[email protected]>
Reviewed-on: https://gitea.com/gitea/docs/pulls/550
Reviewed-by: silverwind <[email protected]>
Co-authored-by: Lunny Xiao <[email protected]>

versioned_docs/version-1.27/installation/with-docker-rootless.md

@@ -66,7 +66,7 @@ server-1  | 2026-03-11T12:57:50.796198843Z /var/lib/gitea/git is not writable
 server-1  | 2026-03-11T12:57:50.796235667Z docker setup failed
 ```
 
-For a stable release you could use `:latest-rootless`, `:1-rootless` or specify a certain release like `:@dockerVersion@-rootless`, but if you'd like to use the latest development version then `:nightly-rootless` would be an appropriate tag. If you'd like to run the latest commit from a release branch you can use the `:1.x-nightly-rootless` tag, where x is the minor version of Gitea. (e.g. `:1.16-nightly-rootless`)
+For a stable release you could use `:latest-rootless`, `:1-rootless` or specify a certain release like `:@dockerVersion@-rootless`, but if you'd like to use the latest development version then `:main-nightly-rootless` would be an appropriate tag. If you'd like to run the latest commit from a release branch you can use the `:1.x-nightly-rootless` tag, where x is the minor version of Gitea. (e.g. `:1.27-nightly-rootless`)
 
 ## Named volumes