Upgrade to MDX 3

[daneah]

The problem

I want to use the latest versions of our dependencies so that I can take advantage of their new features, security, and ecosystem.

The solution

Upgrade to MDX 3

Additional information

This might be a bit involved. Namely:

• MDX > 1 does not allow expressions, so any logic must be in another module and imported, if needed.
• MDX linting seems to get confused about closing tags that are on their own lines—maybe this is improved when all the related packages are upgraded together at the right time.

[daneah]

@satya-achanta-venkata did some investigation and found that Gatsby 5 doesn't yet support MDX 3—this should wait until that changes.