Docs model description

[Sachagobeyn]

@johanvdw :

• First version model description.
• Added page to index.
• Added bs4 to requirements.txt (it was missing for me).
• Proposal to restructure page

[Sachagobeyn]

@johanvdw :

Let's review was available now and discuss:

1. If possible, rename and split 'Watem-module'

   1. Computation erosion map (watem): 1.1 Compute upstream area (now 3.1) 1.2 Compute LS (now 3.2) 1.3. Compute water erosion (now 3.4) 1.5. Compute Tillage erosion (now 3.5)
   2. flanders-tools 2.1. Prepare parcels map (now 1) 2.2. Filter DTM within parcels (now 2) 2.3. Prepare C-factor grid (now 3.3) 2.4 Compute C (now 3.6) 2.5 Complete computation.

2. We can consider to make getting-started much shorter by deleting screenshots and shortly describe what a user need.

[johanvdw]

@johanvdw :

Let's review was available now and discuss:

1. If possible, rename and split 'Watem-module'

I'll do that in a separate PR. I will now make sure that github pages builds so we have http://dov-vlaanderen.github.io/saga-watem/