Split document into per-clause files and a number of fixes

[ronaldtse]

I've split the document into per-clause files and re-arranged the content a bit to fit the required OGC document structure.

Also, I have added a dummy annex for tests because it's referenced but not around.

The model diagram had issues so I also cleaned it up a bit, and used the abstract PlantUML keyword for abstract classes previously denoted as <<abstract>>.

[ronaldtse]

@chris-little I've made some changes to how we encode the terms and definitions.

Change: Abbreviations

Previously, the "Abbreviations" clause was under "Conventions". However, in the new structure it should either be a standalone clause, or under "Terms, definitions and abbreviated terms". I've moved it to the latter for now.

Change: Concept mentions

For terms and definitions, there used to be "anchors" set to the terms for cross references. Metanorma now implements "concept mentions" where all terms can be directly linked by repeating the term itself.

e.g. old:

[[crs_term]]
==== coordinate reference system
...

==== temporal coordinate reference system

<<crs_term,coordinate reference system>> based on a temporal datum

Now becomes:

==== coordinate reference system
...

==== temporal coordinate reference system

{{coordinate reference system}} based on a temporal datum

[.source]
<<iso19111>>

Change: Location of abbreviations

In general, when a term that is a subject of an abbreviation entry is already described in "Terms and definitions", then we keep the abbreviation encoded there, instead of needing a separate line in the "Abbreviations" clause.

This way:

==== temporal coordinate reference system
preferred:[TRS]

{{coordinate reference system}} based on a {{temporal datum}}

The preferred:[TRS] or alt:[TRS] indicates it is a preferred and accepted term respectively.

[chris-little]

@cmheazel Can you review this today please?