[hotfix][docs] Add Analytics GA4 implementation to be prepared for migration

[MartijnVisser]

Google has a new implementation to track website behaviour, called GA4. This will replace the existing UA implementations and is not compatible with the current version of tracking behaviour.

This PR implements the GA4 tracking next to the UA tracking, so that at a later stage the UA tracking can be removed.

See also apache/flink-web#475 and https://support.google.com/analytics/answer/10089681?hl=en

Commit 7b874ea makes sure that the old implementation anonymises IP addresses by default. The new implementation already does this by default. See https://support.google.com/analytics/answer/2763052?hl=en

[flinkbot]

CI report:

• 7b874ea Azure: FAILURE

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[flinkbot]

Thanks a lot for your contribution to the Apache Flink project. I'm the @flinkbot. I help the community
to review your pull request. We will use this comment to track the progress of the review.

Automated Checks

Last check on commit 6b0787e (Thu Oct 14 12:34:39 UTC 2021)

Warnings:

• No documentation files were touched! Remember to keep the Flink docs up to date!

_{Mention the bot in a comment to re-run the automated checks.}

Review Progress

• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❓ 3. Needs [attention] from.
• ❓ 4. The change fits into the overall [architecture].
• ❓ 5. Overall code [quality] is good.

Please see the Pull Request Review Guide for a full explanation of the review process.

Details

The Bot is tracking the review progress through labels. Labels are applied according to the order of the review items. For consensus, approval by a Flink committer of PMC member is required

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot approve description to approve one or more aspects (aspects: description, consensus, architecture and quality)
• @flinkbot approve all to approve all aspects
• @flinkbot approve-until architecture to approve everything until architecture
• @flinkbot attention @username1 [@username2 ..] to require somebody's attention
• @flinkbot disapprove architecture to remove an approval you gave earlier