WEB: Adding governance document to the website
This adds the governance document to the new website.

The rest of the information in the governance repo should already be in the website, so we shouldn't need that repo anymore.

Advantages I see on having this in the website:
- Users can see the governance information from the website, with the same look and feel as the rest, and not navigating to an external github repo
- Simpler (one extra markdown file in the website vs a whole github repo)
- No repetitive information to maintain. The list of maintainers, committees... is now in `web/pandas/config.yml` and can be used in any of the pages of the website. So, in case of changes there will be a single place where to change things.
- I think having it in the main pandas repo will make it easier to keep things updated. There are currently several outdated links in the governance repo (mainly pointing to the repo in `pydata/pandas` instead of `pandas-dev/pandas`).

@jorisvandenbossche you said you saw advantages on keeping the information in a separate github repo, can you comment here about them please.