Use one single Metanorma YAML file

[ribose-jeffreylau]

In part, this is to help fix #61 (and hence #44).

Motivation:
metanorma/metanorma-cli#354 (comment)

The goal is to make glue code for Relaton index files unnecessary. But first, the multiple Metanorma YAML files have to be combined into one, and only split up using liquid templating.

UPDATE:

This task now depends on completion of the following:

• Make site generate able to generate one index HTML for each group of collection (defined by customizable grouping rule) metanorma/metanorma-cli#355