enable tag based trigger

[saraswatpuneet]

Problem

Acceptance Criteria

• [x] Update deploy workflow to use mdbook build
• [x] Should be dependent on CI lint passing
• [x] Should only deploy when tagged with a date based deployment regex ^\d{8}\.\d+
• [x] Update readme to detail the date based deployment versioning. Link to GitHub Issue(s): https://github.com/LibertyDSNP/spec/issues/157

Solution

• [x] ensure the cicd workflow for mdbook triggers on tag creation
• [x] step one should only trigger when date based tag is created via regex checks
• [x] deployment step should not trigger for PRs

Change summary:

• [x] Updated mdbook_ci.yaml to include three conditions above
• [x] Updated ReadMe with instructions to deploy via date tag

Steps to Verify:

1. PR checks should not run deployment step
2. Create a temporary tag without following regex ^\d{8}.\d+, build should not trigger in this case

Test:

• [x] Ran tests to ensure deployment workflow is picking up the correct sha of last commit on a branch

[saraswatpuneet]

@wilwade if it looks good then I can merge and test it out on main

[saraswatpuneet]

also @wilwade I do not think I have permissions to create tags on spec