docs: clarify WordPress 404 page caching

[altis-assistant]

Describe the PR

Clarifies the page caching documentation for 404 responses:

• distinguishes nginx-generated 404s, which use the default 5 minute TTL, from WordPress-generated 404s
• notes that WordPress-generated 404s send no-cache headers and are not cached by default
• adds an example for explicitly caching high-traffic WordPress 404 pages with Cache-Control

Manual verification:

• git diff --check

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For Altis Team Use

Completion Checklist

Does this PR meet our definition of done? See the Play Book Definition of Done

• Has the acceptance criteria been met?
• Is the documentation updated (including README)?
• Do any code/documentation changes meet project standards?
• Are automatic tests in place to verify the fix or new functionality?
  • OR, are manual tests documented (at least on the ticket/PR)?
• Are any Playbook/Handbook pages updated?
• Has a new module release (patch/minor) been created/scheduled?
• Have the appropriate backport labels been added to the PR?
• Is there a roll-out (and roll-back) plan, if required?

[github-actions]

Successfully created backport PR for v23-branch:

• [Backport v23-branch] docs: clarify WordPress 404 page caching #1216

[github-actions]

Successfully created backport PR for v24-branch:

• [Backport v24-branch] docs: clarify WordPress 404 page caching #1217

[github-actions]

Successfully created backport PR for v26-branch:

• [Backport v26-branch] docs: clarify WordPress 404 page caching #1218

[github-actions]

Successfully created backport PR for v27-branch:

• [Backport v27-branch] docs: clarify WordPress 404 page caching #1219

[github-actions]

Successfully created backport PR for v25-branch:

• [Backport v25-branch] docs: clarify WordPress 404 page caching #1220