Link to and summarise Essence docs

[gskorokhod]

See: https://conjure.readthedocs.io/en/latest/essence.html

Need an accessible explanation of what Essence is and where to find docs / examples.
Also, a formal grammar if there is one!

[ozgurakgun]

• Closest thing to a formal grammar is what you see on that page, well at least for a subset of the language.

• Also this paper contains a full spec of the language, except a few things have changed since then (mostly additions). The paper is still a good resource to see the overall idea of the language.
  https://link.springer.com/article/10.1007/s10601-008-9047-y

• Quick google brought up this: https://github.com/eatkins/tree-sitter-ebnf-generator
  As @leiamcalisteryoung's grammar becomes more and more complete, maybe we can extract an ebnf from that?