Adds definitions to block custom view tutorial

[iOvergaard]

📋 Description

Adds definitions to the block custom view tutorial on what the different block types are: block-rte, block-list, and block-grid.

📎 Related Issues (if applicable)

✅ Contributor Checklist

I've followed the Umbraco Documentation Style Guide and can confirm that:

• [x] Code blocks are correctly formatted.
• [x] Sentences are short and clear (preferably under 25 words).
• [x] Passive voice and first-person language (“we”, “I”) are avoided.
• [x] Relevant pages are linked.
• [x] All links work and point to the correct resources.
• [x] Screenshots or diagrams are included if useful.
• [x] Any code examples or instructions have been tested.
• [x] Typos, broken links, and broken images are fixed.

Product & Version (if relevant)

CMS 16

Deadline (if relevant)

ASAP

📚 Helpful Resources

• 🧾 Umbraco Contribution Guidelines
• ✍️ Umbraco Documentation Style Guide