Markdown in docs v1

[tertsdiepraam]

A while ago, I posted some plans for better documentation: #3181

This PR is a another step towards that plan.

After help

uudoc now supports clap's after_help, which is useful for a lot of utils. We might need to tweak what it looks like, but we can figure that out later.

Read directly from markdown file

In #3181, I created a diagram that might be useful in understanding this change. If a file like numfmt.md is available, uudoc will try to read that instead of getting its information from clap. The reason for this is that we don't want to feed clap raw markdown. Instead, we want to render the markdown to something that looks nice in the terminal. For now, I only make one simple change before handing the string to clap: removing all backticks. So now the online docs for expr can look like this:

while the CLI help is:

Print the value of EXPRESSION to standard output

Usage: target/debug/coreutils expr [EXPRESSION]
target/debug/coreutils expr [OPTIONS]


Arguments:
  [expression]...  

Options:
      --version  output version information and exit
      --help     display this help and exit

Print the value of EXPRESSION to standard output.  A blank line below
separates increasing precedence groups.

Notice that there are no backticks around EXPRESSION. This change is not super helpful, but it functions as a proof of concept, I suppose.

Next steps

The next step is adding a full markdown parser and render more complicated markdown. We can play with colors, spacing, etc.. Afterwards, we can open issues to convert all utils to this format.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/tee. tests/misc/tee is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/tee. tests/misc/tee is passing on 'main'. Maybe you have to rebase?