Provide table of contents in pendulum documentation

[Jari27]

• [x] I have searched the issues of this repo and believe that this is not a duplicate.

Issue

The official pendulum documentation is hard to read, hard to search and does not provide a list of functions that can be used. It's more a 'user guide' than official api reference.

The least we we can do to support people using pendulum is to provide a table of contents to the docs, so that people can guess what section they need to read.

[xzmeng]

It already has a ToC, which may be not very noticeable.

[xzmeng]

I agree that an API reference would be very useful.