Check and revise heap analytics tracking for all docs: viz, framework & datasets

[stichbury]

Probably one for @tynandebold

I've been playing around to start pulling in heap data for our new docs subprojects but need adult supervision to make sure it's done correctly:

1. I've not changed the framework docs but please could check the tracking code is OK. I see the dev environment as the primary tag but I think the env_override is to switch production data on for builds that are deployed to stable for RTD. Could you confirm that though (just in case we aren't capturing the right data).

2. Datasets PR: style: Add shared CSS and meganav to datasets docs kedro-plugins#400 <-- Please confirm I've used the right tag and set up properly.

3. Viz PR: Add heap parameter to docs script for analytics #1594 <-- Likewise please check and confirm all is well.

[tynandebold]

I think we can close this as I've verified the docs data is coming through as expected to Heap for each project.