New pandas website
xref: #15556

I've been thinking in more detail about the website, and I think it'll make things much simpler if we keep the pandas website and the documentation separate. My previous proposal was to unify everything, and use nginx rules to show the latest version (`master`) for pages that we want to show the latest version.

I still think that from the user perspective, there should be one single website, with one look and feel, a common header / navigation bar (no matter you're in the website or in the docs). It's easy to make it transparent that we're using different repos/hosts (with different domains or via nginx).

To keep things simple, I'd move from the docs (currently in this repo) to the website repo, everything in the website that doesn't make sense to show outdated (latest stable) versions, including:
- Install
- Roadmap
- Ecosystem
- Contributing documentation

I think this will make things much much simpler regarding Sphinx, will speed up a bit the documentation build, will avoid having to link users to the dev docs (like for contributing), and will simplify things in our workflow and the website set up. The only drawback I see is that we may have to repeat the main template, the css and the logo in both repositories. Those should change very rarely, and we can consider some CI check or automation (download them from one repo to another to build the docs).

For the documentation, I think with this approach will make more sense to create a custom (very simple) theme. This will make things easier and with the bootstrap theme I had some limitations that the maintainers don't seem so keen to address (dropdowns in the top navigation bar, customization in the levels of the sidebar...).

Another advantage is that I think we should be able to move much faster with this architecture when building the new website with the new design, compared to what would be with the Sphinx.

My proposed top-level structure is as follows:
- Home
- Install
- Documentation
  - Tutorials
  - User guide
  - API reference
  - What's new / Release notes
- Community
  - Contributing
  - Roadmap
  - Ecosystem
  - Discuss / Mailing list
  - StackOverflow
- Blog
- Donate

I think the home page should be a visual and concise about us, so users new to pandas can very easily understand what the project is about.

For the blog, I'd mix the posts in the pandas blog, with a planet with the pandas related (tagged) posts of core devs, NumFOCUS, and anyone else we think it's worth.

For the Donate, NumFOCUS works with a new provider, that for what I've seen (I can be wrong), is able to handle the donations in an iframe, and not make users leave the page. I'd also include mentions to the institutional sponsors, comments from individual donors, and things like that.

Once there is agreement in the main structure, and we have a first draft, I'll open separate issues to discuss the exact details of the home, the community page, the blog and the donate page.

CC: @pandas-dev/pandas-core @stijnvanhoey 