docs: Port text deleted in https://github.com/CQCL/hugr/pull/694 .

[cqc-alec]

There was no obvious place to put this text so I created a docs directory.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f2d4c7e) 82.89% compared to head (565fab0) 82.52%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #244      +/-   ##
==========================================
- Coverage   82.89%   82.52%   -0.38%     
==========================================
  Files          34       34              
  Lines        4228     4228              
  Branches     4143     4143              
==========================================
- Hits         3505     3489      -16     
- Misses        536      553      +17     
+ Partials      187      186       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ss2165]

Worth adding a reference + link to the main hugr spec, doesn't make much sense without it.

[ss2165]

thanks 👍