Overview issue: documentation structure

[exaexa]

The docs are more like a collection of ideas now, we should give it a clear tutorial structure.

My idea for the structure is as follows:

• [x] Introduction, what does COBREXA solve and what it does not solve
• [x] Totally primitive example ((down)load 1 model and do 1 FBA)
• [x] Section with tutorials
  • [x] Model loading and conversion
  • [x] Running FBA, FVA and sampling on small models
  • [x] Running FVA on HPCs
  • [x] Modifying and saving the models (Serialize, "export" through conversion to JSON/MAT and saving.)
• [x] Section with advanced tutorials
  • [x] Using modifications to scan for many model properties at once (this still needs to be implemented, but it's hopefully 50LOC :D )
  • [x] Using a custom model structure
  • [x] Writing own reconstructions and modifications and running them on HPC
  • [x] Using the extra information in StdModel to screen through many model variants, e.g. knockouts or something (I guess some of the original tutorials may sink in here)
• [x] Examples (aka backing notebooks)
  • [x] Loading and saving
  • [x] Simple FBA and FVA
  • [x] Sampling and seeing the results
  • [x] Parallel FVA
  • [x] Knockouts
  • [x] Custom models
• [x] Function documentation (REFERENCE, this should correspond to the structure in src/ as much as possible. I'd still separate it into some roughly consistent functions ordered by kinda bottom-up structure)
  • [x] Types
  • [x] IO
  • [x] Analysis functions
  • [x] Sampling
  • [x] Modifications and reconstruction functions
  • [x] Utils+misc

Feel free to edit/suggest.

[exaexa]

cc @laurentheirendt @stelmo

[exaexa]

(This tracks state of what is merged in #164)