feat: Add Attributes and Markdown guidance to template editing

[nicoallgood]

Use-case
When adjusting my template to systematically include meta_description, I found myself wondering what other attributes would be worth specifying in my template. To do so I had to jump back to New Post to double-check the actual Attributes examples.

Suggestion
The same way this list of Attributes is valuable when writing a new post, I think it is equally valuable when editing the template. I therefore suggest to add them on the Edit Template page (and likewise for markdown syntax!).

Sanity checks

• Build/QA done: none
• Disclaimer: First contribution to this project! Unsure if I should have gone through some conventions and/or doing some local pre-validation
• Minimum I could do: run these changes through local browser inspector to get a feel of what it would look like

✨

[nicoallgood]

Oh well I just figured now to double-check for CONTRIBUTIONS.md and understood that Bear Blog is currently not actively accepting contributions. 🤷🏻‍♂️

I hope this PR didn't seem that much low-effort. Feel free to do what you deem best with it!