Add redirect from old board page to flashing

[kprosise]

This ended up being more of a correction to an existing redirect.

Built site, tested redirect locally. Ran linkcheck. No issues

This commit addresses issue FFTK-4740, "fix broken link to board page"

PR Template and Checklist

Please complete as much as possible to speed up the reviewing process.
You may delete items that are not relevant to your contribution.
Readiness and adding reviewers as appropriate is required.

All PRs should be reviewed by a technical writer/documentation team and a peer.
If effecting customers—which is a majority of content changes—a member of Customer Success must also review.

• Run make linkcheck, and add redirects for any moved or deleted pages.
• View HTML in a browser to check rendering.
• follow best practices for commits.
  • Descriptive title written in the imperative.
  • Include brief overview of QA steps taken.
  • Mention any related issues numbers.
  • End message with sign off/DCO line (-s, --signoff).
  • Sign commit with your gpg key (-S, --gpg-sign).
  • Squash commits if needed.