Document the new dimensions campaign group and placement in all guides/faqs/pages

[mattab]

follows up #55

we would want to document the new parameters and dimensions in all guides that mention the other ones such as:

• top 5 results of https://matomo.org/?s=%22pk_medium%22&lang=en
• adding to URL builder: https://matomo.org/docs/tracking-campaigns-url-builder/ (need to mention this requires Matomo 4 in the description)
• the URL builder within Matomo app itself
• any page of developer guide that may mention eg. pk_source or campaign source

[tsteur]

Updated the guide on the website itself. The https://matomo.org/docs/tracking-campaigns/ guide won't need to be updated as we're currently reworking it anyway and will get this integrated there in the new guide.