Add v0.2.0 APIs (Document, Schema, Branch, Streaming), docs, and guides

[thanos]

Implement the first v0.2 slice: document CRUD with streaming, schema frame retrieval, branch management, and an incremental concatenated-JSON decoder. Add comprehensive documentation, guides, and a Livebook demo.

New modules:

• TerminusDB.Document: insert/get/query/replace/delete/stream (plus !/ variants) with graph_type, author/message commit metadata, pagination, and all document query params.
• TerminusDB.Schema: frame/all (plus !/ variants) for class frame retrieval with compress_ids and expand_abstract options.
• TerminusDB.Branch: create/delete/exists? (plus !/ variants) with :from, :organization, :repo overrides.
• TerminusDB.Streaming: split_concatenated/1 (bracket/depth-aware JSON splitter) and document_stream/1 (Stream.resource over Req async responses). Fixed a bug where the stream never halted on :done; added Jason decoding so the stream yields maps, not binaries.

Documentation:

• Added hermetic doctest examples to every public function across all modules (27 doctests total, all using the fake Req adapter pattern).
• Rewrote README Quick Start as a full 11-step walkthrough (create DB, insert schema, insert/query/replace/delete documents, schema frames, branches, streaming, cleanup).
• guides/introduction.md: TerminusDB concepts (documents, schema, branches, queries, indexes/storage).
• guides/migrating-from-sql.md: SQL-to-TerminusDB migration by example.
• guides/overview.md: feature-by-feature walkthrough with runnable examples.
• guides/terminusdb_ex_livebook.livemd: full Livebook demoing every API.
• Updated mix.exs docs extras with all guides under a "Guides" group.

Tests:

• 151 unit tests + 6 properties + 27 doctests, 0 failures (8 integration excluded). 99.2% coverage.
• Streaming tests mock Req.Response.Async with a custom stream_fun to test single-chunk, multi-chunk, split-document, error, trailer, and unknown message scenarios.
• Integration tests for Document, Schema, and Branch lifecycle.

Other:

• Added AGENTS.md
• Updated CHANGELOG with v0.2 Unreleased section.
• Updated .sobelow-conf to ignore SQL.Query false positive on Document.query.
• Updated mix.exs: source_url, description, quality alias (sobelow --exit Low), docs extras, coveralls.json for minimum coverage enforcement.