Document recommended file extension and MIME type docs

[bollwyvl]

📑 Summary

This adds documentations for the recommended file extension and MIME type for mermaid content.

Resolves #3098

📏 Design Decisions

This chooses a terse, but not too terse MIME type.

Once officially documented, the process for getting recognized by the IANA would be the first step to this becoming more widely-recognized, but takes a while to synchronize across all the sources of truth.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added unit/e2e tests (if appropriate)
• 📓 have added documentation (if appropriate)
• 🔖 targeted develop branch

[sidharthv96]

If we're really solidifying the extension, does anyone have any other suggestions?
Will the similarity between mmd and md be an issue?

[bollwyvl]

Thanks for the speedy review!

Will the similarity between .mmd and .md be an issue?

As another data point: presently GitHub will render .mmd and .mermaid files, while GitLab won't render either (perhaps because there is no official guidance). So perhaps the recommendation should also include .mermaid.

[mermaid-bot]

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[sidharthv96]

Procedure for getting IANA registration : https://www.w3.org/2020/01/registering-mediatypes

[bollwyvl]

Thanks: good luck with the IANA registration process! i'll pass on whatever the premium thing is, but good luck on that, too!