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)

tools/watch.sh

@@ -0,0 +1,8 @@
+#!/bin/bash
+set -euo pipefail
+
+make --no-print-directory watch-frontend &
+make --no-print-directory watch-backend &
+
+trap 'kill $(jobs -p)' EXIT
+wait