[Docs] add ranking functions + make the supported table more obvious

[mneedham]

Changelog category (leave one):

• Documentation (changelog entry is not required)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

• Adding examples for row_number, rank, dense_rank
• Adding examples for avg/max aggregation functions
• Making the supported table a bit clearer at a glance
• Formatting the other queries so they're a bit easier to read

[robot-clickhouse]

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Check name	Description	Status
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Docs check	Builds and tests the documentation	✅ success
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[clickhouse-ci]

This is an automatic comment. The PR descriptions does not match the template.

Please, edit it accordingly.

The error is: Changelog category is empty

[nikitamikhaylov]

@mneedham Please don't ignore the PR template https://github.com/ClickHouse/ClickHouse/blob/master/.github/PULL_REQUEST_TEMPLATE.md otherwise tests won't be launched.