[pdal] PDAL Website Migrating to ReadTheDocs

[Howard Butler]

Thanks to Ogi Moore, PDAL will be migrating its homespun documentation infrastructure to ReadTheDocs after the PDAL 2.4.3 release, which is expected before the end of the month. 

You can see a preview of it at https://pdal.dev/ 

One useful feature of the migration is PDAL will now have multi-version docs. Another will be faster documentation builds that are rendered into PR checks with each commit. If you see any significant issues with the new site, please file a ticket.

Howard