Adds gh-pages support

[adamnova]

The issue or feature being addressed

Introduces support for Github Pages for Polly using DocFx.

Details on the issue fix or feature implementation

Addresses #1561

Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (6e69ec3) 83.99% compared to head (06aaf7a) 83.99%.

❗ Current head 06aaf7a differs from pull request most recent head 3ed859f. Consider uploading reports for the commit 3ed859f to get more accurate results

Additional details and impacted files

@@              Coverage Diff               @@
##           docs/gh-pages    #1582   +/-   ##
==============================================
  Coverage          83.99%   83.99%           
==============================================
  Files                280      280           
  Lines               6548     6548           
  Branches            1020     1020           
==============================================
  Hits                5500     5500           
  Misses               839      839           
  Partials             209      209           

Flag	Coverage Δ
macos	83.99% <ø> (ø)
windows	?

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