[apm] Make apm guide URLs apm specific and unified

[simitt]

The apm guide recently moved to https://www.elastic.co/guide/en/observability/current/apm.html. The URLs of the guide should now be following a common naming schema indicating to be part of the apm guide.
For example, the synthetics guide prefixes everything with synthetics- (https://www.elastic.co/guide/en/observability/8.12/synthetics-support-matrix.html).

At the moment this leads to several URL that look like generic observability URLs, but only contain apm specific content, such as https://www.elastic.co/guide/en/observability/8.12/release-notes-8.12.html.

cc @colleenmcginnis

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Once the link to the release notes change, a bit of follow up work must happen so that releases point to the right documentation:

• if change for already released versions, links in github releases need to change (https://github.com/elastic/apm-server/releases/tag/v8.12.0)
• a comment needs to be added to https://github.com/elastic/infra/pull/40558/files, indicating what the new release note URL for apm is

[simitt]

@bmorelli25 I see that this considered for next sprint in January, has there been any update on this?
For now, we move forward with the release note URLs as they are.

[bmorelli25]

Hi @simitt. I have a PR open to address this problem: #3674.

[bmorelli25]

Draft infra PR is up and linked above this comment.