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>
(cherry picked from commit 095fdd691dd1a7d7748372cc73e7708278c80933)

docs/content/development/hacking-on-gitea.zh-cn.md

@@ -201,7 +201,7 @@ REPO_INDEXER_CONN_STR = http://elastic:changeme@localhost:9200
 
 ### 构建和添加 SVGs
 
-SVG 图标是使用 `make svg` 目标构建的，该目标将 `build/generate-svg.js` 中定义的图标源编译到输出目录 `public/img/svg` 中。可以在 `web_src/svg` 目录中添加自定义图标。
+SVG 图标是使用 `make svg` 命令构建的，该命令将图标资源编译到输出目录 `public/assets/img/svg` 中。可以在 `web_src/svg` 目录中添加自定义图标。
 
 ### 构建 Logo