Remove reference to freeseer-docs repo

Also remove trailing whitespace (my editor does that automatically).
Improve getting started section for docs

- Previous installation instructions contained a typo in the package name
  (package name is in titlecase, not lowercase)

- Suggest using requirements.txt to install Sphinx

- Use markdown's reference links for links in getting started section

- Recommend the official install instructions only if pip install failed

- Simplify step 2, remove confusing end of sentence

- Specify the latest version of Sphinx for installation, up to 1.9

  2.0.0 (if ever reached) may contain breaking changes, so we don't want
  to auto-update to that.
Minor improvements to docs readme

- Simpler section headers

- Switch inline links to reference links

- Remove text that wasn't adding much value

[skip ci]