Enable tracking of newsletter-related donations

We want to know if the newsletter has any kind of value in terms of bringing revenue via donations. On the old website there simply was a page identical to the regular donation page that would be logged on Donorbox as a separate campaign.

Following https://github.com/kiwix/web/issues/333#issuecomment-4023062697 @rgaudin hinted that there could be a better/easier way to do it.

Bearing in mind that we want to easily differentiate regular donations from newsletter ones, what are our options?

Review the old donation page and the Donorbox campaign behavior described in the issue, then read issue #333 and its referenced comment. Identify the available options for tracking newsletter donations separately from regular donations; done means an agreed approach that makes the two donation sources distinguishable.

Written by the indexing model from the issue text.