Update documentation/process to clarify when HTML validation takes place

[thomasheritage]

It wasn't clear to me upfront that a document could fail validation even if all issues:

• shown in the browser (in the floating red box)
• given by the validate.mjs script (e.g. installed as a git commit hook)

were resolved.

Additional validation takes place before (and after) the document has been rendered (mainly using html5validator I believe).

It might be helpful to warn the user of this and/or perhaps html5validator could be included as part of validate.mjs (although some practical guidance would be needed on dependencies -- at the minimum Python and Java are needed in addition to Node I think).

Or perhaps the GitHub Actions job (called "build") could be adjusted to make it clearer when a failure is due to the (extra) HTML validation failing (by separating that out as a named step in the Workflow maybe).