This PR contains a first draft of a GDAL command-line cookbook (see #12089). It will need many more examples before getting enough content to be considered for merging, this initial commit is to get agreement on an approach.

The approach aims to:

• avoid repeating examples, by linking to specific examples if appropriate
• link to the examples section of the appropriate tool for more generic questions
• add examples directly to the page if they are not covered elsewhere
• uses traditional and new CLI examples, and PowerShell for Windows users where the example syntax is very different or confusing
• does not cover Python examples - I think this may overwhelm the page, and may be better on a separate page

I went through a few iterations of the page structure (Q&A, bulleted lists), but the use of titles provides a nice page table of contents and linking.

I tried to update the https://github.com/OSGeo/gdal/blob/master/doc/source/_extensions/cli_example.py to allow for global example references, but there were issues with parallel Sphinx builds and having to process each file twice so I stopped. It may just be easier to add :ref:s directly above linked examples.

@dbaston @rouault - let me know your thoughts on the approach and structure.