Add emscripten build instructions to online documentation

[mcbarton]

Description

Please include a summary of changes, motivation and context for this PR.

Currently we only have the emscripten build instructions in a markdown file in the repo. This adds the documentation to CppInterOps online documentation.

Fixes # (issue)

Type of change

Please tick all options which are relevant.

• Bug fix
• New feature
• Requires documentation updates

Testing

Please describe the test(s) that you added and ran to verify your changes.

Checklist

• I have read the contribution guide recently

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.96%. Comparing base (f49ed08) to head (50f878c).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #450   +/-   ##
=======================================
  Coverage   70.96%   70.96%           
=======================================
  Files           9        9           
  Lines        3541     3541           
=======================================
  Hits         2513     2513           
  Misses       1028     1028