Add wasm build instructions to documentation

[mcbarton]

Description

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

This PR copies the wasm build instructions to the online documentation

Fixes # (issue)

Type of change

Please tick all options which are relevant.

• Bug fix
• New feature
• Added/removed dependencies
• Required documentation updates

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.80%. Comparing base (0a3a75e) to head (8259566).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #196   +/-   ##
=======================================
  Coverage   80.80%   80.80%           
=======================================
  Files          19       19           
  Lines         974      974           
  Branches       93       93           
=======================================
  Hits          787      787           
  Misses        187      187           

[anutosh491]

Converting to draft

Once I make the changes required to get xeus-cpp-lite working on xeus-cpp, I shall also change the readme to express the same. We can then get those changes here and merge them too.