feat: `mkdocs` support

[avhz]

It might be nice to generate docs via mkdocs / mkdocs-material / mkdocstrings.

I've added the basic setup, so you can just run:

mkdocs serve --clean

And it will open the docs for you to give you an idea.

Edit: I just noticed some large diffs from my auto-formatter, sorry I didn't spot that prior to opening the PR.

[domokane]

Hi - I am seeing a conflict in financepy/utils/date.py. Why ?

[avhz]

It seems the most recent commit changed that file and now they're conflicting. I can take a look this weekend.

Edit: I fixed it.