docs: use the maintained main-nightly docker tags

The `:nightly` and `:nightly-rootless` tags are no longer published; the
development images are pushed to `:main-nightly` and
`:main-nightly-rootless` instead, so readers following the docker install
pages ended up on an image that has not been rebuilt for over a year.

- point the docker and rootless docker install pages at the
  `main-nightly` tags, 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
- fix the zh-cn/zh-tw claim that `:latest` installs a development
  version, and mention the nightly tags there as the English page does
- render `@dockerVersion@` as `main-nightly` for the next version, so its
  compose snippets no longer reference `gitea:nightly`

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

Co-Authored-By: Claude Opus 5 <[email protected]>

packages/content-loader/src/products.ts

@@ -82,7 +82,7 @@ const variables: Record<string, VersionVariables> = {
     version: 'main-nightly',
     sourceVersion: 'main',
     sourceBranch: 'main',
-    dockerVersion: 'nightly',
+    dockerVersion: 'main-nightly',
     displayVersion: '1.28-dev',
   },
   '1.27': {