docs: mention env var support in apisix.yaml

[kayx23]

Description

Mention env var support in apisix.yaml and add a caution note for exporting vars.

See the below issues for more background info:

Closes #10509
Closes #10530

The info is also mentioned in the Admin API doc but not easy to locate. Personally I only found out after looking for the PR that adds the feature.

From a user perspective, when I search for apisix config env var, this is the doc that comes up first.

Though it seems from the doc title that this doc wasn't originally for this content.

There has been other user that recently mentioned the info could not be located:

https://the-asf.slack.com/archives/CUC5MN17A/p1703518877003939

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)