On beta, adds callout for feedback/beta signup/equivalent legacy pages

[jessicaschilling]

For review by @cwaring and @ericronne on the understanding that this isn't exactly as visually proposed in the original content thread and could likely be more efficiently built, but it gets the job done for something that will only be a temporary part of the beta. Y'all can discuss how much it's worth tweaking. 😉 This completes the work that allows us to close ipfs-inactive/docs#341.

• Adds "this is a beta" callout to the bottom of all content pages (based visually on existing "tip" styles)
• Includes a link to the equivalent legacy page when one exists
• Includes a link to submit feedback (as a generic documentation issue on GitHub in the legacy repo)
• Includes a link to the beta test signup form (using same logic as in the content stub pages)

Example below; note that the "View this page on legacy site" link doesn't appear if there's no equivalent page on legacy.

[ericronne]

I think we'll want to use the tip style i spec'd in this issue once it's been added to the code, but looks merge-able to me!

[jessicaschilling]

Leaving this PR open until @cwaring determines if/whether/how this should be incorporated into a site footer.

[ericronne]

My commit moves the feature suggestion to this legacy callout, but requires additional formatting by Sir @cwaring. 🙏

As of this commit …

Desired design …

To do …

• Add vertical spacing above the callout (should match spacing between major page sections)
• Split the four links into two columns on desktop
• Change the "View this page on legacy site" to regular (thinner) weight font

Related: This issue to remove the feature request from the header