Help: Add 'Synopsis' section to some command docs

Summarize the command signatures in one block at the top of the documentation as is typical in Unix command-line tool manuals. Make the mode keywords links to the corresponding full signature and documentation.

This can be naturally applied to the existing documentation of install, list, and string commands, so do these first. Also re-organize the file command documentation to do this too.

Update cmRST to support rendering the inline links nicely in cmake --help-command output.

Issue: #17948