Refactor `Meilisearch::Index` class

Description

The Meilisearch::Index class is over 1400 lines long and handles too many responsibilities: documents, search, facet search, settings, stats, and compaction. This makes it difficult to navigate, maintain, and review changes.

The class can be split into focused modules, each responsible for a specific area of the API, without breaking the public interface.

Basic example

All public methods continue to work without changes:

index = client.index('movies')
index.add_documents([{ id: 1, title: 'Cloud Atlas' }])
index.search('cloud')
index.settings

Internally, the Meilisearch::Index class would include separate modules:

module Meilisearch
  class Index < HTTPRequest
    require 'meilisearch/index/documents'
    require 'meilisearch/index/search'
    require 'meilisearch/index/settings'
    # ...
  end
end

Each self-contained module would live under the Meilisearch::Index class namespace (e.g. Meilisearch::Index::Documents, Meilisearch::Index::Search):

# lib/meilisearch/index/search.rb
module Meilisearch
  class Index
    module Search
    end

    include Search
  end
end

Read the current Meilisearch::Index implementation and compare its responsibilities with the proposed modules under lib/meilisearch/index/, including search.rb. Group the existing methods by the named areas while preserving the public interface, then run the existing test suite; done means the documented add_documents, search, and settings calls continue to work unchanged.