[DOCS] Add link to APM python LOG_ECS_FORMATTING

[basepi]

@bmorelli25 There's probably a better way to do this than with the full link. Especially because the full link points to master, not current, since current doesn't have it yet.

Closes elastic/apm-agent-python#1007

[apmmachine]

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• Build Cause: Pull request #39 updated

• Start Time: 2021-03-11T16:03:55.548+0000

• Duration: 8 min 11 sec

• Commit: 15c5783

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[bmorelli25]

Unfortunately, there isn't much we can do to make this better. We have attributes for cross-doc python agent links like this, but as you've noticed they only point to current. This link will need to be updated to use the apm-py-ref attribute after the AUTO_ECS_LOGGING feature releases.