add links to GitHub Discussions #27

[pdurbin]

Part of #27

Since I added an icon in the header, here's a screenshot:

Not that I also added a text link to the header. If this is too much we can remove one of them.

I also links to GitHub Discussions on various pages. I'm tempted to add a dedicated page called "Getting Help" (or something) under the User Guide (or Quickstart?) so that it shows up in the main table of contents, which currently looks like this:

[pdurbin]

@raprasad and I just talked about this and made a couple decisions.

• We like the chat icon and will leave it as-is (linking to GitHub Discussions)
• Instead of the external link "GitHub Discussions" we'll change it to a static page called "Contact" where we will list GitHub Discussions.

Update: Done in 9e71670. Screenshots below. Note that "Contact" is its own guide/section so it appears in the table of contents. I wasn't sure how to best word when to use the info account. I plan to add Twitter to the Contact page as part of #40.