Add alt text workshop documentation

[isabela-pf]

We've been running alt text/image description workshops across projects. These events could be run by anyone, so we need to document resources and how to use them. This will include

• [x] Event agenda template
• [x] Working PR template
• [x] Contributing PR info (usually have to work with a project's template)
• [ ] Outline for slides (maybe one day have markdown slides for reuse?)
• [ ] Image description tips/guide
• [ ] FAQs
• [ ] List of roles in the event (and lessons learned by role)
• [ ] Step-by-step guide on running the event

[isabela-pf]

• [x] List of past events

[MarsBarLee]

[ ] Selecting a video conference platform [ ] Promoting on social media