Adds other booking systems beyond eventbrite

[dpshelio]

A year ago or so, Eventbrite started to require a paid plan for events larger than 25 people, even if they were free. Last month, they seem to have reverted that change recently. However, we've found alternative platforms that fulfil our needs and some are Open Source.

The changes suggested provide two alternatives to Eventbrite: pretix.eu and tickettailor. To ease managing them and simplify the content on index.md those sections have been moved to includes like with the installation steps.

There are some bits, like with Eventbrite, that workshop organisers would benefit to know about (e.g., for pretix you've got to enable the widgets on their platform). I'm not sure where to put that type of documentation, or leave it to the person setting it up to find the way around those platforms' documentation. That would require quite a lot of effort, and increase the difficulty to maintain it.

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[froggleston]

Hi @dpshelio ! We are in the process of trialling Pretix to replace Eventbrite. We will be self-hosting the solution as there are some changes and integrations we would want to make between existing Carpentries resources e.g. AMY, and Pretix. Sadly, this won't be a quick process, and full implementation of pretix for events will likely be happening mid- to late-2025.

[dpshelio]

That's great! For now, this PR could be an intermediate step, as it is not using the self-hosted pretix solution, is allowing organisers to use the pretix provided hosted service (and tickettailor)