merge directives into one page about lighthouse-docs

[spawnia]

Resolved by #22 as part of the v2.2 docs

@chrissm79 can you close this?

[spawnia]

I am worried that the list is going to get super long. Also, i think it might make it hard to find a directive for a specific use case.

However, i am a bit surprised that the pages take so long to load and do not seem to be cached. Switching between the sections certainly feels sluggish.

[ctf0]

not really, check lodash docs, all the api methods are listed in the sidebar under different cats, using search simply filters the list

[ctf0]

@spawnia awesome job, btw it could be easier for users to navigate the api if u listed it on the sidebar under separate categories

[spawnia]

It is hard to categorise directives, since some of them have many use-cases. I chose to go for a simple alphabetical listing and plan to add comprehensive guides that deal with a certain aspect of directives.