DOC: add a background section to docs

[aburrell]

Requested feature

It would be nice to have a 'background' section in the documentation that briefly discusses the different coordinate systems, what uses they're good/bad at, and how the secular variation is treated in each of them.

The problem or gap this feature will address

It can be confusing to start using magnetic coordinates, especially when there are so many options.

The desired solution

Add a chapter to the docs. Include this reference:

Quoting @ljlamarche in #115: This article was helpful for understanding what exactly the secular variation code was trying to do. https://earth-planets-space.springeropen.com/articles/10.1186/s40623-021-01507-z

Possible alternatives

Include a section with just useful references for future reading

[ljlamarche]

Laundal and Richmond, 2017 is the best all-around reference I've found for this topic, but it's quite long and detailed and a short overview or summary of the relevant parts may be helpful. This may be related to the discussion on #12 about creating a jupyter notebook tutorial for how to properly work with the Apex base vectors. @asreimer and @klaundal, did anything every come of this?