uucore_procs: extract "about" and "usage" info from new help structure

[cakebaker]

This PR adds the implementation for the new help structure discussed in https://github.com/uutils/coreutils/issues/4368, which no longer uses Usage and About headers:

# numfmt
```
numfmt [OPTION]... [NUMBER]...
```

Convert numbers from/to human-readable strings

The PR

• adds a new help_about macro to get the about text from the help file
• modifies the existing help_usage macro accordingly
• updates all help files to the new structure
• replaces the usage of help_section!("about", "example.md") with help_about!("example.md")

[github-actions[bot]]

GNU testsuite comparison:

Congrats! The gnu test tests/misc/tee is no longer failing!
Congrats! The gnu test tests/misc/timeout is no longer failing!

[tertsdiepraam]

Looks good to me! @sylvestre do you agree with this format?

[sylvestre]

Do we have protections in place in case it is doing unexpected parsing ?

[tertsdiepraam]

Do you want to check the parsed output or do you want the parsing to be more strict? I suppose we could add a check that the output is at least non-empty?

[sylvestre]

i am wondering if making a mistake in the md won't merge the usage & about :)

[sylvestre]

let's see :)