add table header to the notebook cells table

[tonyfast]

i removed the table header a big ago. it was in the thead element which played tricks on some screen readers. this pull request tests how a normal row in tbody works with voiceover.

• adds an action to provide a demo link when a pull request is created.

closes #78

[github-actions]

Exported examples are rendered at https://iota-school.github.io/notebooks-for-all/branch/79/merge/exports/html

[github-actions]

A preview fo the exported examples will be rendered at
https://iota-school.github.io/notebooks-for-all/branch/main/exports/html

[github-actions]

A preview fo the exported examples will be rendered at
https://iota-school.github.io/notebooks-for-all/branch/table-headers/exports/html

[tonyfast]

list and landmark navigation on voiceover ios, but table does not.

[tonyfast]

hmm. i need to check on NVDA but i dont think the headers are adding any value.

[tonyfast]

this is passing automated axe tests and looks qualitatively good on the w3c validator.

merging