[Epic] Improve docs for indexes

Currently, docs for indexes are scattered across multiple places. For example, there is no single place where a reader can find all the examples of working with different types of indexes: how-to information can be found in Concepts, How-tos, API, and so on.

This issue is the next step after https://github.com/tarantool/doc/pull/3562 and suggests the possible solutions for improving docs for Indexes:

1. It would be convenient to have all 'How-to' information for indexes in one place - the Using indexes topic. We can move the following information to this topic:
   • Examples from the API docs: Examples for box.index
   • Examples from space_object:create_index(): Examples
   • Examples from Concepts for RTREE: https://www.tarantool.io/en/doc/latest/concepts/data_model/indexes/#rtree-indexes
   • Examples from Concepts for BITSET: https://www.tarantool.io/en/doc/latest/concepts/data_model/indexes/#bitset-indexes
2. Move all How-to examples to the code_snippets folder and reference them using literalinclude. When possible, make them look uniformly with the current code_snippets/indexes samples.
3. Create new testable samples for all members of the box.index submodule. The https://github.com/tarantool/doc/pull/3562 PR added examples for select and aggr (count, min, max) methods.
4. Try to reuse samples created in the previous step in the CRUD examples topic. To do this, we need to add the testable samples for CRUD operations on spaces (which might duplicate the examples for the primary index to some extent). Index-related examples for spaces can be reused here:
   • space_object API
   • https://www.tarantool.io/en/doc/latest/concepts/data_model/operations/
5. Move the information about iterators from the pairs method description to Concepts because iterators can also be used in other methods (for example, select). Note: currently, there is no link from Iterators to Collations although they are directly related.
6. Add the links from the CRUD API members (for example, select) to the Complexity factors section.

As the result, we'll have a single How-to topic that shows how to work with indexes. This topic should have more clear structure, better distinguish examples for different index types, different CRUD operations, have a separate section for Pagination for better visibility.
Note that currently Google search finds only the Index Concepts topic, so we need to mention the How-to topic in Concepts explicitly.

We also need to take into account the following issues:

• https://github.com/tarantool/doc/issues/2024
• https://github.com/tarantool/doc/issues/1943
• https://github.com/tarantool/doc/issues/2689

Start by reviewing the existing how-to/db/indexes topic, code_snippets/indexes samples, Concepts data_model/indexes and operations topics, the box.index and box_space API pages, CRUD examples, and PR #3562. Done means the index documentation has a coherent How-to structure, reusable testable samples, the requested cross-links, and clearer coverage of index types, CRUD operations, iterators, and pagination.