(maint) use named footnote links in markdown
(maint) use named footnote links in markdown

By using named footnote links we don't have the pain of renumbering
every time we add to/change the footnotes.

Also update the link to the specifications repository.

puppetlabs/cpp-pcp-client,MikaelSmith/cpp-pcp-client,MikaelSmith/cpp-pcp-client,puppetlabs/cpp-pcp-client,MikaelSmith/cpp-pcp-client,MikaelSmith/cpp-pcp-client,MikaelSmith/cpp-pcp-client,puppetlabs/cpp-pcp-client,puppetlabs/cpp-pcp-client