add global TOC to doc index page

[newville]

This adds a Table of Contents to the sidebar for the index page of the docs.
See https://lmfit.github.io/uncertainties/ for results

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.74%. Comparing base (5954d5c) to head (fa5c05e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
- Coverage   95.74%   95.74%   -0.01%     
==========================================
  Files          12       12              
  Lines        1904     1903       -1     
==========================================
- Hits         1823     1822       -1     
  Misses         81       81              

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[wshanks]

This is an improvement to me. Especially on mobile (or just making the browser window narrow), it is nice to have the navigating on the side.