Update maintainer guide

Checklist to collect many small fixes.

Installing the application:

* [x] 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
* [x] 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](https://docs.google.com/presentation/d/13e2q9zbF6glZmlcdC_2CrSdAUPkLZTn-2RS64hmWL9c/edit?usp=sharing) which has point form notes of all the different installation options