Add optional description field for Contact Block

#### Is your feature request related to a problem? Please describe.

I would love to have the ability to add a description for fields in the Content Block, to provide additional information for users.

#### Describe the solution you'd like

I believe that having a way in the block to enter a description below the form would be best. An example is as follows:


This could allow basic formatting, such as links, bold, etc. We could leverage the `RichText` Core component for this purpose.

#### Describe alternatives you've considered

There's not a good way to currently do this. Right now, the only way to add context for users is to add footnotes essentially, which is not easy to read. Currently most sites I see that need this just embed Google Forms.

#### Additional context

Current FE output: