add tutorial on developing an extension

[andrewbanchich]

Check for existing issues

• Completed

Describe the feature

The current docs don't provide much information on developing extensions - they go over directory structure, but then basically end with "then just use zed_extension_api and draw the owl write the extension".

I looked through the existing extension implementations but it isn't super clear to me what I should be doing for mine, what's applicable and what isn't, etc.

It would be very useful if you could add an example tutorial to the docs for this.

If applicable, add mockups / screenshots to help present your vision of the feature

No response