links inside How to/Contribute (as a table of contents)

[MarPetra]

Code Snippets
http://pgrouting.org/docs/howto/contribute.html#code-snippets
Get Involved
http://pgrouting.org/docs/howto/contribute.html#get-involved
Fork pgRouting on Github
http://pgrouting.org/docs/howto/contribute.html#fork-pgrouting-on-github

all this links returns to "how to/ contribute" (there is already a table of contents inside)
http://pgrouting.org/docs/howto/contribute.html

• are we keeping?

[cvvergara]

Do not move, this is pending after you finish the others

[cvvergara]

Remove dead links