move some scripts from 'build' to 'tools' directory, misc refactors (#29844)

- Move some scripts from `build` to new `tools` dir. Eventually i would
like to move all but let's do it step-by-step.
- Add dir to eslint and move the files into vars.
- Update docs accordingly.
- While updating docs I noticed we were incorrectly having `public/img`
path still in a few places. Replace those with the current
`public/assets/img`.

---------

Co-authored-by: Nanguan Lin <nanguanlin6@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

.gitignore

@@ -58,7 +58,7 @@ cpu.out
 /data
 /indexers
 /log
-/public/img/avatar
+/public/assets/img/avatar
 /tests/integration/gitea-integration-*
 /tests/integration/indexers-*
 /tests/e2e/gitea-e2e-*