Add links to example `.pre-commit-config.yaml`s

[asottile]

The documentation could use with more links to some examples. (Adding issue on behalf of @ivanlei)

[philipmulcahy]

The section starting with "Adding pre-commit plugins to your project" is pretty unclear.
Maybe it could be worded differently and show usage of more/most of the "keywords"?

s/The pre-commit config file describes:/The following directives/options control which hooks are applied to your project and how they behave:/

Also, the table should clarify at what level in the file the different words are relevant - for example "sha" applies one level higher than "id" - you could either split the table into two groups, one for the top level (per repo) and one for the lower level (per hook). I reckon a BNF for the config file format is overkill, but the current setup is not clear enough.

[asottile]

@philipmulcahy (yes, I'm very late to the party) separating out repo / sha just happened in #138 so now it's a little more obvious (I hope!)