Docs: added table of contents for the API reference

[halfzebra]

Thanks for Parsimmon! 👍

Not sure if this makes a lot of sense taking into account #230

I usually find the table of contents to be useful in the docs, so here's a PR that adds ToC.

See the example here.

[coveralls]

Coverage remained the same at 100.0% when pulling 3dd1004 on halfzebra:patch-1 into f1d2dee on jneen:master.

[wavebeem]

When #230 happens we can look into automatically generating the ToC. Until then, this PR adds a lot of value for everyone.

Thanks a lot for this PR 😄