document puppet more and alternatives to monorepo

i particularly want to go through and summarize:

• #29387, #29663 (closed), #30770 (closed) - alternative deployment options
• #31226 - validation hooks
• LDAP integration and lookups
• lookup mechanisms (Hiera's lookup(), Puppet's puppetdb_query(), puppetdbquery's query_nodes(), LDAP and exported resources (!!))
• external data sources (Nagios, let's encrypt certs, auto-ca certs, hopefully didn't miss anything)
• disaster recovery (hope really really hard that backups work)
• add a glossary
• install procedures
• design overview
• git-subrepo review

I particularly did some research on the first item in the list and need to braindump this.

Update: the list above contains direct links to the sections that have been added or improved.