Add optional index description functionality

[daffidwilde]

Fixes #312.

It was such a small addition I went ahead and made the PR. Apologies that I'm pulling into main...

I haven't written any specific tests for this functionality, although I am happy to do so if you can point me in the right direction.

I have tested it locally on the project I mentioned in the issue.

Let me know what you think 😄

[has2k1]

Ideally, in the Builder class this should stop at adding builder.description and then the renderer would generate the markdown (ref #314).

[daffidwilde]

Thanks for reviewing this @has2k1

I have tried to use the custom index page functionality (as suggested in #312) to get what I need to no avail, and I can appreciate that the Builder class is not the place for this change.

It sounds like #314 is the way to go and I will close this PR 😃