Add heap parameter to docs script for analytics

[stichbury]

Description

My attempt to set up Heap. Needs to be verified by someone who understands what they're doing.

Development notes

QA notes

Checklist

• Read the contributing guidelines
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added new entries to the RELEASE.md file
• Added tests to cover my changes

[stichbury]

@astrojuanlu @tynandebold Please can I check on the progress of this in my absence, if any?

[tynandebold]

No progress from me. I've just added it to our new sprint, so we're tracking it now and will get to it soon.

[tynandebold]

This is working as it should be for me. I noted in a comment we're not switching between dev and prod environments, and I don't think we need to.

Here are the events coming through: