WEB: Add governance page to the website
I was having a look at the [pandas governance repo](https://github.com/pandas-dev/pandas-governance) and seems most things there are duplicated from the website. The list of core developers, the code of conduct, the sponsors, are all in the website, and in my opinion presented in a clearer and more accessible way.

The only thing that is missing is the governance document itself, moving it we can remote that repo. I think it makes more sense to move it here, for few reasons:
- To avoid duplication and inconsistencies, and having to maintain things in two different places
- To have more visibility and easier to access by having it in our public web
- We may want to update the governance document, I think it makes things easier to have the PRs with ammendments to the doc here with the rest of the PRs of the project, instead of in a separate repo

For now I move the `governance.md` file without changes. I think the file can benefit from few changes (updating the title, links...), but I prefer to do it in follow up PRs, so there is visibility of what's being changed.

Somehow related to this PR: @MarcoGorelli, I see in the governance repo we've got Gousto as a pandas sponsor, but it's not in the website (and the logo is not in the home page). Should it be added?

Hey - I'll be leaving Gousto in September to join Quansight, so it should probably be removed Thanks @datapythonista 