Document `index_object:info()` method

Since: 3.9.0
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_index/

Returns engine-specific information about an index.
Unlike index_object:stat(), which reports runtime counters, info()
is meant for static details about how an index is organized internally.
At the moment, the only engine that implements it with real content is
memcs (Enterprise Edition).

Parameters:

• index_object – an object reference to the index.

Return value:

• Type: table
• Engine-specific information about the index.

Example:

tarantool> box.space.test.index.pk:info()
---
- columns:
    column1:
      layout:
        dict: false
        null_rle: false
      compression:
        type: none
    column2:
      layout:
        dict: false
        null_rle: true
      compression:
        type: none
    column3:
      layout:
        dict: true
        null_rle: false
      compression:
        type: lz4
        acceleration: 1

Also link this page from https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/create_index/#index-opts-layout
Requested by @Gumix in https://github.com/tarantool/tarantool-ee/commit/cd301f44185cd83f4a9523b115b6ec849901c6a7.

Start with the box_index reference page and the create_index page's index-opts-layout section named in the issue. Document index_object:info() with its parameters, return value, engine-specific behavior, and example, then link the method page from the layout section. Done means both pages contain the requested documentation and link.