Update the documentation of the C code

[ttusar]

This outdated documentation needs to be updated: https://numbbo.github.io/coco-doc/C/:

• It contains old links to COCO
• It is missing some observer parameters

In addition, the instruction on how the C code documentation is compiled should be added to the development readme.

[ttusar]

Also, there seems to be some missing pictures and math code that is not correctly displayed: #28

[ttusar]

The instructions to produce the documentation files can be found here