docs(redis): document the Web Console sentinel password (cherry-pick of #62)

[chideat]

Cherry-pick of #62 (release-5.1 → master). Applied with git cherry-pick -x; no conflicts, and the resulting content is identical to the release-5.1 commit.

Why

Creating a Sentinel instance in the Web Console now presents two password tabs — Set Redis Password and Set Sentinel Password. The Sentinel tab has Set Password enabled by default with its fields required, so a new Sentinel instance normally carries a Sentinel credential of its own. The docs had no name for that credential and no mention of the tab, and the client examples only ever set the Redis password.

Existing instances are unaffected: the tabs render only while an instance is being created or restored, never while updating.

What changed

docs/en/release_notes.mdx — a v5.1.0 entry, Sentinel Password Enabled by Default in the Web Console: the tabs, the default-on behaviour, the warning icon on the tab label if you click Create with the fields empty, the spec.sentinel.passwordSecret vs spec.passwordSecret split, and the note that existing instances are untouched.

docs/en/functions/10-create-instance.mdx

• Web Console table: a new Set Sentinel Password row under Connection Configuration (rowspan 3 → 4), and a clarification on the existing Set Default Password row that for Sentinel it lives on the Set Redis Password tab and covers the data nodes only.
• Sentinel CLI tab: a note that the example sets neither password, with the Secret shape (key password, 8–32 characters, per ValidatePasswordSecret) and the spec.sentinel.passwordSecret reference.

docs/en/how_to/access/10-sentinel.mdx — a two-password table in Authentication Requirements, and the sentinel password added to all four client examples.

TERMINOLOGY_CANDIDATES.md — logs Sentinel password / Set Sentinel Password for confirmation.

Client fields — verified, not assumed

Client	Field	Verified against
go-redis	FailoverOptions.SentinelPassword	redis/go-redis sentinel.go — passed as Password in sentinelOptions()
Jedis	.password(...) on the sentinelConfig DefaultJedisClientConfig (the 5-arg JedisSentinelPool already splits master vs sentinel config)	redis/jedis docs
Lettuce	withSentinel(String host, int port, CharSequence password), replacing withSentinel(RedisURI.create(...))	overload read from RedisURI.java upstream; Lettuce docs confirm the URI password applies to data nodes only
Redisson	.setSentinelPassword(...)	redisson/redisson configuration docs — falls back to the Redis password when unset

Notes for the reviewer

• "Mandatory" is a Web Console default, not an API rule. The Set Password toggle is an ordinary switch the user can turn off, and the webhook only validates a non-empty passwordSecret — so the docs say "enabled by default … unless the toggle is deliberately turned off".
• Whether the tab itself is new in 5.1 or only the default-on behaviour could not be confirmed from source (the frontend history is squashed into its initial split commit). The heading hedges accordingly.
• docs/en/functions/70-backup-restore.mdx is not touched, though restore shows the same tabs.
• Unrelated: master is still missing the cherry-pick of docs(redis): correct the cross-datacenter replication API surface #59 (docs/dr-api-corrections), so TERMINOLOGY_CANDIDATES.md and release_notes.mdx differ from release-5.1 here for reasons predating this PR.

Verification

yarn lint — 0 errors, 0 warnings. yarn build — succeeds.