Add test workflow checking html build w/o deploy

[tmorshed]

Adding testing workflow so that we won't need to rely on docs builds.

Bare minimum:

• Builds on pushes before merging and without deploying
  (for pushes to main/dev or any branches with patterns ['dev/*', 'feat/*', 'fix/*', 'hotfix/*', 'enh/*', 'misc/*'].

Next steps:

• Check the types in this test (as implemented by @kevinkadak).
• Ninja level: Check outputs of notebooks/examples to see if they're in a good range.

[JohnGriffiths]

c.f. test bits in eegnb test yml

https://raw.githubusercontent.com/NeuroTechX/eeg-notebooks/master/.github/workflows/test.yml

(note, probably better ones in other repos...)