Add an overview section to the ProgLearn documentation

[bstraus1]

Reference issue

Closes #424

Type of change

Documentation

What does this implement/fix?

Add an overview section (as in the Verde documentation)

[PSSF23]

@bstraus1 This change does not make sense to me. We have the overview already as the index page, so why don't you add your changes there?

[bstraus1]

@sampan501 asked for a separate overview section (that is not a landing page) and follows the format of the Verde documentation.

I agree that it has some redundancies, but also appreciate the point @sampan501 made saying that this will say the essential info not covered in the landing page.

[PSSF23]

@bstraus1 I see. Then substantial differentiation should be made. The true overview should not contain indices or badges, and the landing page would remove the "overview" terms. No sentences should be repeated.

I can modify them if you wish.

[bstraus1]

Sure. Or, let me know what to do. Either way works.

[PSSF23]

@sampan501 Does the current overview satisfy your needs?