Docs: move aggregate function docs to source (#4)

[Blargian]

Moves aggregate function documentation into source code (#4)

Changelog category (leave one):

• Documentation (changelog entry is not required)

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

...

Documentation entry for user-facing changes

• Documentation is written (mandatory for new features)

[clickhouse-gh]

Workflow [PR], commit [a73b12a]

Summary: ❌

job_name	test_name	status	info	comment
Stateless tests (amd_binary, ParallelReplicas, s3 storage, parallel)		failure
	02319_lightweight_delete_on_merge_tree	FAIL	cidb, issue
Stateless tests (amd_debug, AsyncInsert, s3 storage, parallel)		failure
	02319_lightweight_delete_on_merge_tree	FAIL	cidb, issue
Stateless tests (amd_debug, distributed plan, s3 storage, parallel)		failure
	02319_lightweight_delete_on_merge_tree	FAIL	cidb, issue
Stateless tests (arm_binary, parallel)		failure
	02319_lightweight_delete_on_merge_tree	FAIL	cidb, issue
Integration tests (amd_binary, 4/5)		failure
	test_merge_tree_s3/test.py::test_merge_canceled_by_s3_errors[node-broken_s3_always_multi_part]	FAIL	cidb
BuzzHouse (amd_debug)		failure
	Logical error: 'Inconsistent AST formatting: the query: (STID: 1941-1bfa)	FAIL	cidb, issue