add lead paragraphs to index.mds

[julia-pfarr]

Your idea

Every upper-level section (=tab) should have a lead paragraph as their index.md. See lead paragraph wikipedia

[ericearl]

To clarify for anyone else that sees this issue, this was discussed at the maintainers meeting and brought about because some pages have good lead paragraphs and others have less good lead paragraphs.

So this issue is especially here because the pages navigation may change soon, and that may require more lead paragraphs or different lead pargraphs.

Did I capture that right, @julia-pfarr?