Add CI link checker

[knmcguire]

It would be nice to have a more automatic way to check all the links in aerial robotics landscape.

• https://pypi.org/project/mkdocs-linkcheck/ (found by @TOTON95

This was something that we would have liked to do for #69 but there were some false positives there, but let's experiment with it more before actually integrating it.

[TOTON95]

Hey @knmcguire, this is looking good so far, did some changes in #72. Take a look whenever you have a chance 🙌🏼

[knmcguire]

Let's keep this open until we found a way to make the CI fail more reliably, but #72 is already a great step forward.