Add ChoiceBySchemeDocumentLoader for scheme-based document loading

[anatoly-scherbakov]

Summary

• Add ChoiceBySchemeDocumentLoader, which dispatches to per-scheme loaders so a local file: document can resolve a remote https: @context
• Export the loader, cover dispatch/fallback/errors with tests, and document it with an offline composed-loader example
• Add an example_data macro and tabbed sourced JSON-LD on the ChoiceByScheme and File loader pages

Stacked on #301 (file-document-loader). Closes #300.

Test plan

• pytest tests/test_choice_by_scheme_document_loader.py
• make test
• make lint
• make docs-build
• Confirm docs page tabs show Example + sourced JSON-LD for ChoiceByScheme and File loaders

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
lib/pyld
canon.py	218	4	98%	27, 180, 565–566
context_resolver.py	103	8	92%	78, 137, 151, 197, 203–205, 229
iri_resolver.py	141	3	98%	311–312, 322
jsonld.py	2517	166	93%	258–262, 311, 348, 353, 363, 380–395, 431–432, 474, 482, 502, 510–511, 619–624, 704–705, 720–721, 781–786, 811–812, 827–828, 838–839, 865, 906, 916, 922–926, 937–938, 981, 988, 1053, 1076, 1098–1100, 1110–1113, 1128, 1151, 1163–1166, 1262, 1274–1277, 1297, 1326, 1359, 1363, 1400, 1443, 1462–1465, 1478, 1527, 1630–1637, 1664–1666, 1692–1695, 1871, 1909, 2215, 2346, 2355–2362, 2456, 2483, 2507, 2515, 3106, 3151, 3170, 3235, 3288, 3291, 3354, 3439, 3473, 3481, 3759, 3764, 3766, 3951–3955, 4066, 4108, 4171, 4406, 4694, 4826, 4860, 4889–4892, 4918–4919, 5102, 5166, 5193, 5267, 5269, 5287, 5385, 5434, 5666, 5668, 5707, 5709, 5718, 5860, 5988, 6075, 6195–6209, 6271, 6273, 6341, 6554, 6560–6564, 6603, 6606, 6696–6697, 6706
lib/pyld/documentloader
aiohttp.py	71	22	69%	29–39, 68, 74, 86, 98–106, 118–121, 138, 153–156
file.py	36	1	97%	49
requests.py	52	5	90%	57, 71, 83–91
scheme_directed.py	15	1	93%	38
TOTAL	3481	210	94%

Tests	Skipped	Failures	Errors	Time
1734	37 💤	0 ❌	0 🔥	2m 57s ⏱️