Adds hottest posts endpoint + misc.

[mattrltrent]

• Adds the /posts/hottest/ endpoint.
• Adds steps showing how to view the local MongoDB in README.md.
• Adds /posts/hottest/ route documentation to openapi.yaml.
• Adds a separate created_at field to the Post type.

Closes #7.

[minitech]

Is there a reason to separate yesterday? I’m thinking a /hottest?date=2023-02-12 might be simpler.

[mattrltrent]

Is there a reason to separate yesterday? I’m thinking a /hottest?date=2023-02-12 might be simpler.

I'll consolidate the two queries (past-date and yesterday) into just a general date then!

And, is using milliseconds since unix epoch still a valid way to query? Or, as you wrote, should it be in the format 2023-02-12 (I found that was tricker to implement with the DateTime types available).