Updated documentation.

I mainly removed backlog items, either done or moved to GitHub issues.

Some small things have been fixed too (typo, formatting, ...)

An architecture schema has been added, and you'll need to use https://www.draw.io/ to view it.
Mainly fixed and completed the AT grammar.

- updated the AT parser documentation
- fixed a little the test input for the parser

AT Grammar:

- block nodes are now flagged as "block"
- changed term "element" to "node": more generic, and consistent with HTML
- some unnecessary code/rules have been removed
- changed some precedence in rules for better accuracy and consistency too
- completed the terminals encapsulation in nodes for many rules
- fixed CDATA content parsing (made it optional)
- finished implementing the parsing of expressions (with modifiers)
- fixed some little quirks (like allowing spaces at wrong places, setting properties with nodes, ...)