[DOC] Missing documentation on Aggregations

[hdhalter]

What do you want to do?

Request a change to existing documentation

Tell us about your request. Provide a summary of the request and all versions that are affected.
We have some documentation on aggregations, but we're missing some. Here is what we have https://opensearch.org/docs/latest/aggregations/. Here is what we're missing:

Metric aggregations

• Geo-centriod Add geo-centroid and weighted average aggregations documentation #7613
• Median absolute deviation - Add doc review changes to contributor's blocked PR #6808 #7265
• Weighted average Add geo-centroid and weighted average aggregations documentation #7613

Bucket aggregations

• Auto-interval date histogram Add auto-date histogram aggregations documentation #7630
• Children Add children aggregation documentation #7645
• Composite
• Parent
• Rare terms

Pipeline aggregations

• Moving function
• Percentile bucket
• Serial differencing

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

[dblock]

[Triage -- attendees 1, 2, 3, 4, 5, 6, 7]