Improve styling of tables in docs

I noticed today that [some pages](https://docs.djangoproject.com/en/dev/topics/http/shortcuts/#django.shortcuts.redirect) in the documentation use tables:

I find the styling a bit hard to read:

1) The table headers are not visually separated (no border, no horizontal margin)
2) The table has no bottom margin

Start with the linked documentation page for django.shortcuts.redirect and inspect how its tables are styled. Compare the result with the reported issues: headers should be visually separated with a border and horizontal spacing, and the table should have bottom margin.

Written by the indexing model from the issue text.