Add benchmark files to docs

[mborland]

Closes: #163

[cppalliance-bot]

An automated preview of the documentation is available at https://164.int128.prtest3.cppalliance.org/libs/int128/doc/html/index.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.95%. Comparing base (fee3b43) to head (ae0eacc).
Report is 19 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #164   +/-   ##
=======================================
  Coverage   98.95%   98.95%           
=======================================
  Files          28       28           
  Lines        4108     4108           
  Branches     1022     1022           
=======================================
  Hits         4065     4065           
+ Misses         41       40    -1     
- Partials        2        3    +1     

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[cppalliance-bot]

An automated preview of the documentation is available at https://164.int128.prtest3.cppalliance.org/libs/int128/doc/html/index.html