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 80.72%. Comparing base (0a3a75e) to head (0e7c667).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #196      +/-   ##
==========================================
- Coverage   80.80%   80.72%   -0.08%     
==========================================
  Files          19       19              
  Lines         974      970       -4     
  Branches       93       93              
==========================================
- Hits          787      783       -4     
  Misses        187      187              

see 1 file with indirect coverage changes

see 1 file with indirect coverage changes

[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.

[anutosh491]

LGTM. Merging