Change and unify markup for (poly)variants and records

[dbuenzli]

Tables with fixed width columns don't scale and can lead to unreadable definitions (example).

We agreed with @trefis that it was better to render them as we do for module member: each member/case being in its own div with a properly labelled internal div for the definition (which can be made to wrap like function definitions do) and another div for the doc string.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. The main purpose of this is to keep the issue tracker focused to what is actively being worked on, so that the amount and variety of open yet inactive issues does not overwhelm contributors.

An issue closed as stale is not rejected — further discussion is welcome in its closed state, and it can be resurrected at any time. odoc maintainers regularly check issues that were closed as stale in the past, to see if the time is right to reopen and work on them again. PRs addressing issues closed as stale are as welcome as PRs for open issues. They will be given the same review attention, and any other help.

[dbuenzli]

See #614 for a proposal.