MemCS index aggregates and scanner statistics

Since: 3.6

MemCS indexes were populated with their aggregates and scanner statistics.
They can be found in index:stat(). The statistics are:

• aggregates.total_size - memory occupied by the whole skip index.
• aggregates.block_count - amount of blocks in skip index.
• aggregates.minmax_size - total size of minmax aggregates in all blocks.
• aggregates.bloom_size - total size of bloom aggregates in all blocks.
• scanner.rows_skipped - amount of rows skipped with skip index.
• scanner.rows_scanned - amount of rows scanned with ArrowStream.

Skipped and scannned statistics are cumulative and show amount of
scanned/skipped rows for the whole index lifetime.

NB: note that reads and skips that happened in read view will be accounted
only when that read view is closed.
Requested by @drewdzzz in https://github.com/tarantool/tarantool-ee/commit/9e03b5e42d4fb9648c52892115115e9306291351.

Start by locating the MemCS index documentation and the index:stat() entry point. Document the listed aggregate and scanner statistics, including their cumulative behavior and the read-view closure note. Done means the statistics are explained in the appropriate documentation location and match the issue text.