feat(cockpit): retention updates

[nerda-codes]

• Added new retention periods (from 1 day to 5 years) in the "concepts" page under the "Retention" concept
• Added new FAQ questions => "What is Scaleway data?" and "What is custom data?" with explanation for both
• Added info banner about new pricing updates in FAQ, and "How to change data retention period"
• Added question in FAQ "How am I billed for increasing data retention period" with custom retention pricing examples for 10 days and 3 months in a collapsable bar.
• Added info that Scaleway does not support the Grafana agent
• Added new concepts: "custom data" and "Scaleway data"
• Changed wording from "Tokens" to "Cockpit tokens"
• Deleted concepts previously agreed upon with @sfinet-scw

I am adding a screenshot of how the banner looks below:

This is a screen recording of how the FAQ question with the collapsible example looks:

https://github.com/user-attachments/assets/c402ece4-551d-49de-818e-b5caa85cf00f