Update changelog docs for CDN TTL details

[lcawl]

This PR updates the changelog documentation (e.g. https://docs-v3-preview.elastic.dev/elastic/docs-builder/tree/main/contribute/changelog) related to issues I've hit when doing recent Serverless releases.
In particular, it adds more details about:

• The amount of time you need to wait between uploading a file to the CDN and using it in a bundle.
• The ways you can force the bundle to use local files instead of CDN files
• The reasons why you might choose to have your changelogs in both the repo and the CDN

[Mpdreamz]

why is this an hour @cotti ?

[Mpdreamz]

We shouldn't document folks manually uploading changelogs on the command line.

Everything should be automated through our github actions and the timing of publishing these should not be arbitrary.

[lcawl]

I think the need for that will persist for the ad hoc releases and things like known issues, security issues, feature announcements, etc that aren't ncessary tied to a PR or release. AFAIK those aren't uploaded automatically yet.

[Mpdreamz]

This is a bug, lets zoom in

[cotti]

Heads up: the "wait up to an hour" behavior this PR documents is being removed at the source. elastic/docs-infra#355 disables CloudFront caching entirely for the changelog-bundles CDN (the 1-hour default TTL was the whole delay — the scrubber Lambda sets no Cache-Control, so every object, including registry.json and entry YAML, was cached for the full hour).

Once that lands, the only delay between upload and public visibility is the private-bucket → SQS → scrubber-Lambda pipeline, which is seconds — so the TTL notes here ("minimum 60 seconds", "wait at least an hour after last-minute changelog updates before bundling") will be obsolete and worth rewording to describe just the scrubber pipeline delay. The entry-sourcing / --force-local documentation is unaffected and still useful for not-yet-uploaded local files.