Add Docs/html to Git Ignore

Docs/html holds the files generated by Doxygen.  It should be ignored by
Git.
Upgrade to Doxygen 1.8.6 from 1.7.3

Upgrades Doxygen configuration files to 1.8.6.  The upgrade results in
breaking changes (can't use original configuration) so all Doxygen files
were regenerated and updated as needed.

The Doxyfile shows a large number of changes, but most are the comments
and new settings created by the Doxygen auto-upgrade process.
Otherwise, some obsolete settings were removed and the logo was set to
none since its source image no longer exists in the repository.

Where appropriate the configuration files were updated to link to the
new Git repository.  This is the only significant document content
change.
Sync Doc Home Page Content with GitHub Home

Updates the content of the documentation home page to match the content
of the GitHub repository home page.  This includes the new Recast
introduction image, premake4 based build info, etc.
Add Doc Build Info To ReadMe File

Adds some information on building the documentation to the documentation
ReadMe file.