Document Redis events store for Enterprise

[johanstokking]

Summary

This documents enabling the Redis events store that is used by the Network Operations Center.

References https://github.com/TheThingsIndustries/lorawan-stack-support/issues/1222#issuecomment-2722560117

Changes

• Configure the events configuration section; the default is to use the in-proc internal backend, which does not work with NOC

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Run Locally: Verified that the docs build using make server, posted screenshots, verified external links. Test with HUGO_PARAMS_SEARCH_ENABLED=true if style changes will affect the search bar.
• New Features Marked: Documentation for new features is marked using the new-in-version shortcode, according to the guidelines in CONTRIBUTING.
• Style Guidelines: Documentation obeys style guidelines in CONTRIBUTING.
• Commits: Commit messages follow guidelines in CONTRIBUTING, there are no fixup commits left.