For date, move help strings to .md file

[chusteven]

Description

#4368

Testing

Existing tests still pass

cargo test
...
test result: ok. 2139 passed; 0 failed; 25 ignored; 0 measured; 0 filtered out; finished in 23.66s

Manual debug build shows same as main branch

@ coreutils [stevenchu]$ ./target/debug/coreutils date -h
Print or set the system date and time

Usage: ./target/debug/coreutils date [OPTION]... [+FORMAT]...
./target/debug/coreutils date [OPTION]... [MMDDhhmm[[CC]YY][.ss]]

Arguments:
  [format]

Options:
  -d, --date <STRING>     display time described by STRING, not 'now'
  -f, --file <DATEFILE>   like --date; once for each line of DATEFILE
  -I, --iso-8601 <FMT>    output date/time in ISO 8601 format.
                           FMT='date' for date only (the default),
                           'hours', 'minutes', 'seconds', or 'ns'
                           for date and time to the indicated precision.
                           Example: 2006-08-14T02:34:56-06:00
  -R, --rfc-email         output date and time in RFC 5322 format.
                           Example: Mon, 14 Aug 2006 02:34:56 -0600
      --rfc-3339 <FMT>    output date/time in RFC 3339 format.
                           FMT='date', 'seconds', or 'ns'
                           for date and time to the indicated precision.
                           Example: 2006-08-14 02:34:56-06:00
      --debug             annotate the parsed date, and warn about questionable usage to stderr
  -r, --reference <FILE>  display the last modification time of FILE
  -s, --set <STRING>      set time described by STRING (not available on mac yet)
  -u, --universal         print or set Coordinated Universal Time (UTC)
  -h, --help              Print help information
  -V, --version           Print version information

[chusteven]

One question I had was what the point of the src/uu/date/date-usage.mkd file was though 🤔 and if anything should be done in this PR to consolidate (if desired)

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/tail-2/inotify-dir-recreate is no longer failing!
GNU test failed: tests/misc/timeout. tests/misc/timeout is passing on 'main'. Maybe you have to rebase?