docs: add ifeval for 7.10 release

[bmorelli25]

Summary

This PR fixes a documentation link that will break when 7.10 becomes the current stack version. It conditionally changes the link location of the Logs app docs based on the stack version defined in github.com/elastic/docs (here).

ifeval::["{branch}"=="7.9"]
APM to the {apm-app-ref}/xpack-logs.html[Logs app].
endif::[]
ifeval::["{branch}"!="7.9"]
APM to the {observability-guide}/monitor-logs.html[Logs app].
endif::[]

Admittedly, this isn't pretty, but it's the cleanest way to ensure this link continues to work for users who click it before and after the 7.10 release.

Tested by building locally with :branch: overridden to 7.9, 7.10, and master.

Target

This PR will need to be backported to 1.x.

Related issue

• 7.10 broken links observability-docs#215
• Update current version for 7.10 docs#1994

Follow-ups

After the release of 7.10, I will backport a fix that removes the conditional code.

[apmmachine]

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Build stats

• Build Cause: [Pull request #1004 opened]

• Start Time: 2020-11-03T21:43:53.240+0000

• Duration: 3 min 52 sec

[bmorelli25]

Thanks for the quick review! I'm going to merge this and run a full rebuild on our release PR to make sure everything works correctly before backporting

[bmorelli25]

Confirmed that this approach works. Initiating operation backport.