Fix broken links in documentation

[aedwardg]

Several of the links in the documentation were broken or pointed to the
wrong place (mostly due to re-organized files and changed headings).

All links in the docs should be up to date after this PR.

Code review checklist

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly
• Changelog(s) has/have been updated, as needed (see CHANGELOG.md, no need
  to update for typo fixes and such).

[coveralls]

Coverage remained the same at 91.044% when pulling 4e3991f on aedwardg:fix-doc-links into 042c7e1 on igordejanovic:master.

[igordejanovic]

Looks good. Thanks!