Add block pattern categories' descriptions to the REST API and update the descriptions

[youknowriad]

Related #41379

What?

In #41379 we implemented the new patterns inserter but we left out of the addition of the descriptions to the block pattern categories as a follow-up.

This PR adds these descriptions.

Note I've just added placeholder descriptions for now. Please can you help provide the right descriptions for each category.

Note 2 I know that we want to change the categories, can we leave that out of the current PR and just focus on addition the descriptions for now.

Testing Instructions

1- Open the patterns inserter
2- Notice that you can see "descriptions" under the category title when you open the panel.

[jasmussen]

Nice! Works as intended:

We obv. need some descriptions for each of these categories, and #44501 is related there, CC: @WordPress/gutenberg-design. I guess we need those in place to land this, right?

[youknowriad]

Note 2 I know that we want to change the categories, can we leave that out of the current PR and just focus on addition the descriptions for now.

As noted on the PR description, I was hoping to avoid to wait for you all to update the existing descriptions... can we just come up with descriptions for the current categories to land this PR as is first? An alternative would be to not add descriptions to all current categories and only add to one or two as an example and leave the rest for you all when you rework the categories.

[jameskoster]

'Featured' is kind of ambiguous (I'm still not entirely sure myself) and could use a description.

Query would also benefit from some elaboration since it's a bit technical. Perhaps something like: "Display post summaries in lists, grids, and other layouts"?

Ideally Headers and Footers should only appear when editing templates where the context makes them fairly obvious.

I'm not sure the other categories really need descriptions.

[youknowriad]

To make sure this PR is in a mergeable state, I've removed all the placeholder descriptions, so all categories are without descriptions except "query" where I used @jameskoster's suggestion above.

We can now land the PR and this will allow you to tweak the categories and their descriptions as you wish.

[jasmussen]

Wonderful. I went ahead and renamed Query to Posts per this discussion, and I think it looks good. The fact that there aren't any descriptions in any of the others is fine, now the ability to add them is there, and we can do so in a followup as the remaining parts of 44501 get worked out. Thank you Riad!