Combine redundant sections of contributing doc

[vreuter]

This combines the redundant sections of the contributing doc, regarding how to build the documentation and serve it locally with Hoogle. This would close #1777 .

[dnadales]

Not all of the instructions were properly preserved.

[vreuter]

Not all of the instructions were properly preserved.

@dnadales can you clarify what's improperly preserved? In the diff, the pieces of information I see missing in the new version concern

• running the documentation script
• the local host address and port number

But those pieces of information are still in the document, and under the same heading.