Move the tldr.zip missing warning from runtime (uudoc.rs) to compile
time (build.rs). This ensures the warning is printed only once during
the build, instead of 100+ times when make calls uudoc separately for
each utility.
Fixes#9940
Adjust the formatting of the options and examples sections in the
uudoc output to use Markdown headers:
- Change `<h2>Options</h2>` to `## Options`
- Change `Examples` to `## Examples`
- Add regression test to prevent reverting to HTML headers
Ruiyang Wang
Haowei Hsu
Shay Elkin
n4n5
Aaron Ang