Add link to rgb3gen2 flashing instructions

[kprosise]

Link added to supported boards page.

Built html, checked that link showed and functioned as intended, No issues found.

This commit addresses FFTK-3472 "RB3G2: link to flashing instructions"

PR Template and Checklist

Please complete as much as possible to speed up the reviewing process.

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Overview

Why merge this PR? What does it solve?

Checklist

• Run spelling and grammar check, preferably with linter.
• Avoid changing any header associated with a link/reference.
• Match tone and style of page/section.
• Run make linkcheck.
• View HTML in a browser to check rendering.
• Use semantic newlines.
• follow best practices for commits.
  • Descriptive title written in the imperative.
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  • Mention any related issues numbers.
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Comments

Keeping it simple; shows up in TOC for board flashing instructions, links to our external flashing instructions. This will make it easy if the instructions are ever added directly to the docs.

[doanac]

Docs for 7e6ebfa are browsable at: https://ci.foundries.io/projects/fio-docs/builds/2772/docs/artifacts/html/index.html

[vanmaegima]

@kprosise lgtm, is the long term idea to move this into our docs or to keep it in the external git?

[kprosise]

@kprosise lgtm, is the long term idea to move this into our docs or to keep it in the external git?

Unsure what the plan is.