Add link to each implementation sig repo

[breedx-splk]

Fixes #3528.

Preview here https://github.com/breedx-splk/community/tree/link_repos#implementation-sigs

A few caveats/notes:

• Some implementation SIGs have multiple repositories. This just links the first one. I assume if one is not "correct" someone will reorder the order to their liking.
• Browser has no repos listed and kinda sticks out. 🤷🏻
• @trask wanted it to not have horizontal scroll, and it still looks ok to me (but in fact, the spec sigs table above already has a small scroll...doh! 😸 )

[maryliag]

Browser has no repos listed and kinda sticks out.

there is a repo, so maybe you can add it to this PR? https://github.com/open-telemetry/opentelemetry-browser

[maryliag]

as long as CI is passing, LGTM

[breedx-splk]

<nudge>

[trask]

had an alternative thought: #3579

[breedx-splk]

closing in favor of #3579