Slight edits to intro caggs sections

[Loquacity]

Description

These edits kill a couple of birds with one stone:

• The aggregation concept sections of the GSG were a little confusing, so it hopefully clears up some of that confusion
• We don't tend to mention that caggs are based on hypertables, so this detail has been added

Links

Partial: https://github.com/timescale/docs-private/issues/170

Writing help

For information about style and word usage, see the style guide

Review checklists

Reviewers: use this section to ensure you have checked everything before approving this PR:

Subject matter expert (SME) review checklist

• Is the content technically accurate?
• Is the content complete?
• Is the content presented in a logical order?
• Does the content use appropriate names for features and products?
• Does the content provide relevant links to further information?

Documentation team review checklist

• Is the content free from typos?
• Does the content use plain English?
• Does the content contain clear sections for concepts, tasks, and references?
• Have any images been uploaded to the correct location, and are resolvable?
• If the page index was updated, are redirects required
  and have they been implemented?
• Have you checked the built version of this content?

[github-actions]

Allow 10 minutes from last push for the staging site to build. If the link doesn't work, try using incognito mode instead. For internal reviewers, check web-documentation repo actions for staging build status. Link to build for this PR: http://docs-dev.timescale.com/docs-caggs-editing-lana

[Rajakavitha1]

LGTM!

[iroussos]

Thank you @Loquacity, great initial update!

I added a few thoughts / comments - not everything is a clocking comment, I just added a bunch of thoughts in there; yours to think about where things would be better shown or if they are really needed.

[Rajakavitha1]

LGTM