DEV: (cmds) add INCREX command page

[dwdougherty]

Redis 8.8 feature

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Note

Low Risk
Low risk docs-only change: adds new command page and updates keyspace notification docs without affecting runtime code.

Overview
Adds a new INCREX command page documenting atomic increment/decrement with optional bounds (LBOUND/UBOUND), overflow policies (FAIL/SAT/REJECT), and expiration controls (EX/PX/EXAT/PXAT/PERSIST/ENX), including examples and reply shape.

Updates keyspace notification documentation to describe which events INCREX emits, and adds a new railroad syntax diagram asset at static/images/railroad/increx.svg.

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[github-actions]

DOC-6515

[github-actions]

Staging links:
https://redis.io/docs/staging/DOC-6515/commands/increx
https://redis.io/docs/staging/DOC-6515/commands/increx/
https://redis.io/docs/staging/DOC-6515/develop/pubsub/keyspace-notifications/

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

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^{Security scan by Jit}

[cursor]

Cursor Bugbot has reviewed your changes and found 3 potential issues.

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^{Reviewed by Cursor Bugbot for commit 68edfd0. Configure here.}

[andy-stark-redis]

Language LGTM

[dwdougherty]

TYVM, @andy-stark-redis!

[dwdougherty]

@LiorKogan I think I addressed everything. Please take another look and approve.

[CLAassistant]

All committers have signed the CLA.

[dwdougherty]

@sundb Please sign the CLA.