feat: backlinks from booking page

[therobrob]

• added index "booking" to all operator pages
• modified related-partial
• added backlinks from booking page to operator page

Because the index is on the operator pages, we need an different logic for related-partial than before.

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[github-actions]

Hi there! 👋

Thank you for your contribution to the FIP Guide! 🚀
We appreciate your effort in making rail staff travel information more accessible. 🚄

Checklist before merging:

• Added a description to the Pull Request
• Checked the License of new pictures (non-commercial use without attribution)
• Modified content in English
• Modified content in German
• Modified content in French

[MoritzWeber0]

I'm not sure if tracking the booking platforms in another place is a good idea. I'm sure that the list will get out-of-sync at some point. Instead, we could automatically derive the ticket offices by iterating over the operators and check if the raw content contains booking id="%s". It's less performant during build time, but given the limited number of operators it shouldn't be an issue. What do you think about the idea?

[therobrob]

This seems to be an alternative. I wonder if the booking options change that often so the list will get out-of-sync.
I’ve discussed the new index with @lenderom, so I’m interested to hear his opinion too :)
If we agree, the mapping mechanic could be changed.

[lenderom]

@therobrob Thanks for implementing this ❤️