Add a button to share tutorial achievement on Bluesky

[sarah11918]

📚 Subject area/topic

tutorial - share achievement

📋 Suggested page

https://docs.astro.build/en/tutorial/6-islands/3/

📋 General description or bullet points (if proposing new content)

We currently have options to share a social media post about finishing the tutorial on its final page. However, it only includes sharing options for Twitter and Reddit:

We would happily accept a PR for adding a similar button to share on Bluesky! 🦋

You can find the lines of code where this is located on the last page of the tutorial.

Note: as with all new content to the docs, please only add this in English! We have amazing translators who will carefully translate and add to their own language pages.

🖥️ Reproduction of code samples in StackBlitz

No response