Add centralized documentation for event tracking

What does this MR do?

This takes a more holistic approach to getting frontend and backend tracking documentation together.

Some of the technical details are not 100% accurate yet, but they're in the direction I'm trying to take this. I'm using this as an opportunity to do some document driven development and get feedback in that process instead of doing the MRs directly in the code at the moment. The main objectives are to consolidate the documentation into a single place and to get the layers consistent with one another / more consolidated (event vs. action, and context differences, etc.)

This MR was originally on CE at gitlab-foss!32117 (closed)

Related issues / MRs

• CE - respect DNT settings: gitlab-foss!32030 (merged)
• CE - use cookies for session: gitlab-foss!32236 (merged)
• EE - use cookies for session: !15661 (merged)
• CE - improve tracking performance: gitlab-foss!32669 (closed)
• EE - improve tracking performance: !15759 (merged)
• About - update tracking taxonomy: gitlab-com/www-gitlab-com!28783 (merged)
• EE - switch to AsyncEmitter: !16575 (merged)

Moving docs to a new location?

Read the guidelines: https://docs.gitlab.com/ce/development/documentation/index.html#changing-document-location

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• [-] Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ and ee/app/views (for GitLab EE) directories.
• [-] Make sure to add redirect_from to the new document if there are any Disqus comments on the old document thread.
• [-] Update the link in features.yml (if applicable)
• If working on CE and the ee-compat-check jobs fails, submit an MR to EE with the changes as well (https://docs.gitlab.com/ce/development/documentation/index.html#cherry-picking-from-ce-to-ee).
• Ping one of the technical writers for review.