docs: repoint two links that 404

[lesnik512]

Why

Three links in published surfaces are dead:

Link	Where	Status
faststream.airt.ai	README.md first paragraph, docs/index.md first paragraph	404
modern-di.readthedocs.io	docs/usage/messaging-service.md	404

FastStream moved off airt.ai. faststream.ag2.ai is live and is already what
modern-di-faststream's README uses. The modern-di link should be
modern-di.modern-python.org — the canonical per-project docs URL under the org's
AGENTS.md naming rule.

These sit in the opening sentence of the README and of the docs home page, so they are
about as visible as a broken link gets here.

Design

Three one-line replacements. No prose changes.

Non-goals

Not auditing other external links — the weekly check in modern-python/.github covers
that, and these three are all it currently reports for this repo.

Verification

• Both replacement URLs return 200; both old hosts return 404.
• No occurrence of either dead host remains in the tree.
• lychee --offline still 202 OK / 0 errors, and just docs-build passes.

Worth noting how these were found: the scheduled external check caught them, and the PR
gate structurally could not — no diff in this repo ever touched those lines. That is the
split #65 was arguing for, demonstrated.

[github-actions]

Benchmark gate

✅ gate passed

scenario	msg/s	delete/msg	WALrec/msg	WALB/msg	fpi	upd	del	dead_tup
consumer/w1/b10	984	1.000	6.79	916	242	5000	5000	10000
consumer/w1/b100	1001	1.000	6.82	920	243	5000	5000	10000
consumer/w2/b10	1173	1.000	6.77	944	242	5000	5000	10000
consumer/w2/b100	1360	1.000	6.72	955	243	5000	5000	10000
consumer/w4/b10	1043	1.000	6.82	995	261	5000	5000	10000
consumer/w4/b100	1547	1.000	6.86	1038	244	5000	5000	10000
consumer/w1/b100/tfbs100	5435	0.010	6.07	1116	243	5000	5000	10000
producer/w1/b100	2057	0.000	3.04	584	0	0	0	0

Gated (fails the build): delete_calls + tuple counters (upd/del/ins) + the producer's insert_calls, exact; select_calls within +2; wal_records within a 10% band. msg/s, WAL bytes and total calls are informational (timing/FPI noise).