[2pt] Add example about working with varbinary fields

Product: Tarantool
Audience/target: developers
Root document: https://www.tarantool.io/en/doc/latest/book/box/data_model/
SME: ?

This might lead to restructuring the Indexes section, which is a separate task.

"How to save and load binary fields" is a frequently asked question by customers. There are no mentions of the right ways of using varbinary fields in the documentation.

An example of example:

tarantool> box.schema.create_space('test')
---
- engine: memtx
  before_replace: 'function: 0x010c252e98'
  on_replace: 'function: 0x010c252e60'
  ck_constraint: []
  field_count: 0
  temporary: false
  index: []
  is_local: false
  enabled: false
  name: test
  id: 512
- created
...
tarantool> box.space.test:create_index('primary', {unique=true, parts = {1, 'unsigned'}})
---
- unique: true
  parts:
  - type: unsigned
    is_nullable: false
    fieldno: 1
  id: 0
  space_id: 512
  type: TREE
  name: primary
...
tarantool> mp = require'msgpack'
---
...

tarantool> s = '\xc4\10aaaabbbbccccdddd'
---
...

tarantool> m = mp.encode(s)
---
...

tarantool> box.space.test:insert({1, m})
---
- [1, !!binary ssQKYWFhYWJiYmJjY2NjZGRkZA==]
...

tarantool> dg = require'digest'
---
...

tarantool> dg.base64_decode(mp.decode(box.space.test:select(1)[1][2]))
---
- !!binary aaaabbbbccccdddd
...

Also more information about using indexes with binary fields, limitations and typical patterns will be nice. The example above may need some beautification.

Start with the data model documentation at the linked root document and review its Indexes section alongside the provided Tarantool console example. Document how to save and load varbinary fields, including the example's encoding and decoding flow, and clarify relevant binary-index limitations or patterns if they fit the scope.