Add domain support for directives

[alcarney]

• This starts working towards Add proper support for domains #71, by making directive completions domain aware
• The std and "default" domain directives are suggested without the namespace prefix
• The primary_domain config option is respected.

Also

• Start adding implementation notes to the docs
• Refactored code again so that rather than structuring code according to lsp features (completions etc.), it's structured according to rst language features (directives, roles etc)
• Cache sphinx app instances in the sphinx fixture, with intersphinx hitting the network on initialization it makes a big difference to test duration