Allow linking comments to blocks

Currently, the page's comments are not linked to blocks. In this situation, it is difficult to understand the context of these comments.

For example, if my page has 30 blocks, I don’t know which block John is referring to.

The solution to this problem would be to allow users to link a comment to a block. This way, it would be easier for other users to understand the context of the comments.

Begin with the page comments and blocks described in the issue, then inspect how the notebook UI represents each. Done means a user can attach a comment to a specific block and other users can identify that block when reading the comment. No test or file is named, so trace existing comment and block behavior before choosing coverage.

Written by the indexing model from the issue text.