[MNT] Updated help text and category explanations of the annotation page

[rmanaem]

• Closes Categorization and annotation pages should include proper help text #617

Checklist

• PR has an interpretable title with a prefix ([ENH], [FIX], [REF], [TST], [CI], [MNT], [INF], [MODEL], [DOC]) (see https://neurobagel.org/contributing/pull_requests for more info)
• PR links to GitHub issue with mention Closes #XXXX
• Tests pass
• Checks pass

For new features:

• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

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[surchs]

nice PR @rmanaem! Thanks! I like the use of bullets from the explanation sentences.

One 🍒 and then 🧑‍🍳