Links in new docs script

[johnSchnake]

What this PR does / why we need it:
This script was written to consider the root README as the source
of truth but I think that was before we realized that the docs
for the site have slightly different linking requirements.

As a result, the appropriate place to take the README from is the
master docs.

Which issue(s) this PR fixes
Fixes #887

Special notes for your reviewer:
To test this, I'd do the following:

• checkout this branch
• run the script to generate the 'new' docs for a phony version: ./scripts/update_docs.sh v0.15.999
• commit the change locally (so we can look at the diff later)
• add back the line I removed
• run the same command again ./scripts/update_docs.sh v0.15.999
• run git diff to look at the difference.

You'll see some redundant things (it adds to the TOC and a few other files, but the script is meant to be just for 'new' versions of the docs).

The main thing to see is that the diff should show just a small change in the links. It used to show full URLs for doc pages but it should now be showing relative URLs/paths.

NOTE
I'm going to add a second commit showing what the 'new' docs look like. When approved I'll remove that commit before merging.

Release note:

NONE

[codecov-io]

Codecov Report

Merging #892 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #892   +/-   ##
=======================================
  Coverage   47.29%   47.29%           
=======================================
  Files          75       75           
  Lines        5034     5034           
=======================================
  Hits         2381     2381           
  Misses       2515     2515           
  Partials      138      138

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[zubron]

Thanks for including the instructions on how to check this locally! 👍