Documentation fixes and additions

[isabela-pf]

In reviewing the repo, I've found areas that need clarification or additional information to better document hosting community calls. This PR is in draft because I expect to find more gaps that need fixing as I go.

So far, this PR includes

• Deletion of to-do.md which was an outdated version of the list that became the PR template.
• Adding guidelines on commercial product presentations to faqs.md.
• Creating promotion-examples.md to hold examples of past posts used for announcing community calls. This can help give an idea of tone and what information to include when promoting calls. It is a work in progress.

Please let me know if you see any errors or have other review comments at any point. Thanks! 🌻

[isabela-pf]

Judging by the timestamps, this got away from me. I'm going to merge what I have so I can make other updates as needed.