Add cookbooks to docs

[gassmoeller]

This PR removes the directory doc/sphinx/user_manual/cookbooks and replaces it with a soft link to the directory cookbooks. This way the cookbooks remain in a top level directory, but also appear in the online documentation. Now we can add markdown files (and other files like images) in the cookbooks like I have done here for the 3D Cartesian rift cookbook. This also adds a minimal example for how cookbook documentation can look like.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a01bf98) 93.54% compared to head (8ddab11) 93.55%.
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #558   +/-   ##
=======================================
  Coverage   93.54%   93.55%           
=======================================
  Files          92       92           
  Lines        6340     6343    +3     
=======================================
+ Hits         5931     5934    +3     
  Misses        409      409           

see 3 files with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a01bf98...8ddab11. Read the comment docs.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.024 ± 0.003 (s=452)	1.023 ± 0.004 (s=430)	-0.1% .. +0.1%
Slab interpolation curved simple none	1.026 ± 0.004 (s=463)	1.026 ± 0.003 (s=417)	-0.1% .. +0.1%
Spherical slab interpolation simple none	1.267 ± 0.004 (s=364)	1.266 ± 0.012 (s=349)	-0.2% .. +0.1%
Slab interpolation simple curved CMS	1.098 ± 0.003 (s=416)	1.098 ± 0.004 (s=406)	-0.1% .. +0.1%
Spherical slab interpolation simple CMS	1.859 ± 0.006 (s=238)	1.859 ± 0.009 (s=249)	-0.1% .. +0.1%
Spherical fault interpolation simple none	1.274 ± 0.004 (s=353)	1.274 ± 0.013 (s=356)	-0.2% .. +0.2%
Cartesian min max surface	2.390 ± 0.018 (s=174)	2.392 ± 0.019 (s=205)	-0.2% .. +0.3%
Spherical min max surface	7.350 ± 0.021 (s=56)	7.349 ± 0.015 (s=69)	-0.2% .. +0.1%

[jdannberg]

Other than the directory structure and the two small comments, this looks good to me!

[jdannberg]

Suggested change

	The relevant part of the world builder file looks like this:
	The relevant part of the World Builder file looks like this:

[jdannberg]

Suggested change

	This section contains self contained cookbooks on how to design different geodynamic setups. Folders contain World Builder parameter files, grid output files, and optionally markdown files that are incorporated into the online documentation and explain particular features about the setup.
	This section contains self-contained cookbooks on how to design different geodynamic setups. Folders contain World Builder parameter files, grid output files, and optionally a doc folder that contains relevant figures and markdown files that are incorporated into the online documentation and explain particular features about the setup.

Or if you have a good idea for how not to repeat 'contained' that would be even better.