[2pt] Restructure box.stat page

[veod32]

Product: Tarantool
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_stat/
SME: TBD

Details

Currently, there's a single landing-like page for the box.stat submodule reference. All the submodule functions with all the details are describe in a flat list which is quite a mess and really not convenient to read and use.

Needs to:

• Create a separate page for each function.
• On the main page, there should be just an introduction text and ToC navigating to the pages of the functions.