- 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>

### Before

```
$ make watch
bash build/watch.sh
make[1]: Entering directory '/Users/silverwind/git/gitea'
make[1]: Entering directory '/Users/silverwind/git/gitea'
GITEA_RUN_MODE=dev go run github.com/cosmtrek/air@v1.43.0 -c .air.toml
rm -rf public/js public/css public/fonts public/img/webpack public/serviceworker.js
NODE_ENV=development npx webpack --watch --progress
```

### After

```
$ make watch
GITEA_RUN_MODE=dev go run github.com/cosmtrek/air@v1.43.0 -c .air.toml
NODE_ENV=development npx webpack --watch --progress
```

* Clean the "tools" directory. The "tools" directory contains only two
files, move them.
* The "external_renderer.go" works like "cat" command to echo Stdin to
Stdout , to help testing.
* The `// gobuild: external_renderer` is incorrect, there should be no
space: `//gobuild: external_renderer`
* The `fmt.Print(os.Args[1])` is not a well-defined behavior, and it's
never used.
* The "watch.sh" is for "make watch", it's somewhat related to "build"
* After this PR, there is no "tools" directory, the project root
directory looks slightly simpler than before.
* Remove the legacy "contrib/autoboot.sh", there is no
"gogs_supervisord.sh"
* Remove the legacy "contrib/mysql.sql", it's never mentioned anywhere.
* Remove the legacy "contrib/pr/checkout.go", it has been broken for
long time, and it introduces unnecessary dependencies of the main code
base.

* Add 'make watch'

This combines frontend and backend watch into a single command that runs
them in parallel on on SIGINT terminates both.

Termination is not super-clean but I guess it does not have to.

* move to tools/, trap more signals, remove gnu-specific flag

* simplify

Co-authored-by: techknowlogick <techknowlogick@gitea.io>