Update docs to 4.0.0 changes

[mielvds]

This PR's add some updates to the docs specific to the switch to 4.0.0

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
lib/pyld
canon.py	279	15	95%	35, 46, 49–54, 78–79, 508, 530, 594, 604, 658–659
context_resolver.py	103	8	92%	78, 137, 151, 197, 203–205, 229
iri_resolver.py	141	3	98%	311–312, 322
jsonld.py	2495	174	93%	244–248, 297, 334, 339, 349, 366–381, 417–418, 460, 468, 488, 496–497, 605–610, 690–691, 706–707, 767–772, 797–798, 813–814, 824–825, 851, 897, 906–912, 928–929, 971, 978, 985, 1050, 1075, 1097–1099, 1109–1112, 1127, 1150, 1162–1165, 1261, 1270–1273, 1293, 1322, 1355, 1359, 1392–1397, 1410–1412, 1422–1424, 1467, 1486–1489, 1502, 1551, 1654–1661, 1688–1690, 1716–1719, 1895, 1933, 2239, 2370, 2379–2386, 2480, 2507, 2531, 2539, 3133, 3183, 3202, 3267, 3320, 3323, 3386, 3471, 3505, 3513, 3791, 3796, 3798, 3993–3995, 4144, 4207, 4446, 4734, 4866, 4900, 4929–4932, 4954–4955, 5138, 5202, 5229, 5303, 5305, 5323, 5421, 5470, 5702, 5704, 5743, 5745, 5754, 5896, 6024, 6111, 6231–6245, 6307, 6309, 6377, 6604, 6610–6614, 6653, 6656, 6746–6747, 6756
util.py	72	3	96%	43, 116, 136
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	3478	232	93%

Tests	Skipped	Failures	Errors	Time
1842	29 💤	0 ❌	0 🔥	3m 1s ⏱️

[anatoly-scherbakov]

Maybe turn this into a runnable example, as a Python file under examples?

[anatoly-scherbakov]

Same as above

[anatoly-scherbakov]

And here

[anatoly-scherbakov]

Mre examples! Here and below, if you think that'd be a good thing