Setup URL forwarding with plain HTML files

[leouieda]

We were using subdomain forwarding (calendar.fatiando.org) for some hard
to remember URLs. This unfortunately doesn't work with HTTPS, leading to
some broken links and ugly warnings from browsers about potential
danger. Replace those with a simple HTML redirection structure in
subfolders. For example, fatiando.org/calendar now redirects to the
Google calendar link. Same for fatiando.org/slack and
fatiando.org/good-first-issue. There are instructions in the README
for adding new forwards.
A caveat is that we now have to be careful with the subfolders we create
here. But that's probably not going to be a problem since this website
is intentionally not very elaborate (it links to other resources).

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.