install.rst: remove broken links and improve formatting

[anbj]

Minor improvements.

Why are building instructions in the wiki and not in the docs?

[seisman]

Building instructions are in https://github.com/GenericMappingTools/gmt/blob/master/BUILDING.md, not in the wiki.

[anbj]

Ah, and that document isn't referenced in the technical reference, right?

Does that mean that all references to the wiki (build related) should instead link to BUILDING.md?

[seisman]

Ah, and that document isn't referenced in the technical reference, right?

Yes

Does that mean that all references to the wiki (build related) should instead link to BUILDING.md?

Yes.

Originally, the installation instructions were in the INSTALL.md file in the root directory of the repository and were not included in the documentation, so the linking to BUILDING.md works. However, a few months ago (#7885), we decided to move the installation instructions to the docs but forgot to update the links to BUILDING.md.