Rework References/Services sub-section

[cmgrote]

• [ ] Helpful to group OMAS, OMIS, OMES, OMVS under a sub heading of "Registered Services" and the rest under "Fixed Services" to show which are optional in the platform and which are always present. I am assuming these pages will become these services "Home page" that the Egeria diagnostics -eg audit log messages - point to. These links are in the code in the component description enums.
• [x] Something weird with the Governance Zone page https://odpi.github.io/egeria-docs/concepts/governance-zone/ in that it does not have any menus

[cmgrote]

@mandy-chessell on the first point, to confirm, this?

• Fixed Services:
  • Common Services
  • Platform Services
• Registered Services:
  • OMRS
  • OMAS
  • OMES
  • OMIS
  • OMVS
• Governance Services:
  • Integration Daemon Services
  • Engine Host Services
  • Open Lineage Services

(The second one is what happens if the page itself does not appear anywhere in the overall site navigation -- have fixed for my next PR.)

[cmgrote]

See new structure as of #37 (should match my description above) -- OK now, or further changes needed?