Add alt text/image description event templates

[isabela-pf]

In running multiple alt text/image description events, I've developed some templates to help me create a consistent experience. This PR adds

• Event agenda template
• Working PR template
• Contributing PR template

as was discussed as a part of #3.

Some things I'd like to edit before merging:

• Add a brief note for how each template should be used
• Add a note on the contributing PR template that it is important to follow a project's PR template if they have one. This info can be combined with those in such cases.
• Proofread and make sure everything is accurate.

[isabela-pf]

All edits made, so this will be merged! 🌻