Update maintainer guide

[jodygarnett]

Checklist to collect many small fixes.

Installing the application:

• Installing from the installer is empty!
• Installing form WAR file is actually about Tomcat, and does not provide configuration direction
• Installing from source code - Move content to GitHub repo README.md and link to it
• Installing remote index - can we just say Elastic Search
• Configuring the database - does this database have a name? Can we have headings for H2 and PostgreSQL
• data directory, should this be before installing?
• loading templates and sample data <-- is this a dup with quickstart?

Recommendation:

• Maintenance Guide can discuss trade offs during installation, quickstart should not
• Adopt a numbered step approach or template (especially for installation)

See:

• GeoNetwork Orientation which has point form notes of all the different installation options