Handbook: Add German entity page (pay slip, tax certificate provider)

Why is this change being made?

The German entity has specific details and gotchas which are spread around Slack threads and different issues. Today, I missed that CloudPay is the SSoT as pay slip provider, and Arbeitnehmer Online being deprecated once again.

This MR adds a German entity sub page, copying the tips from @leipert and adds the new learnings from Slack (internal). It also adds a note on accessing the tax documents required from German employees.

Once persisted in the handbook, we can also keep track of announcements older than 90 days, and document more entity specific requirements, like the mail footer requirements.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the "Maintained by" section in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)