README.MAPS was unreadable and wildly outdated, with many dead links,
references to web sites by name but *without* any link, and references
to external tools that are no longer available.
Further, it was unstructured and basically an accretion of random
map-related information of various levels of usefulness --- some only
to new users, some only to those building the code, some only to
developers.
With this commit, all of the remaining information that was in
README.MAPS and that wasn't pointing at inaccessible web resources has
been migrated. I've added the internal details portion to "Advanced
Map Topics" and moved all the stuff about CAD polygons (which isn't
really a map topic at all, but an operating topic) to its own page.
