Assistant documentation in Altinn docs

[ErlingHauan]

Release documentation for the assistant when we go to beta.

A draft (only accessible by direct URL) has been made here:
https://docs.altinn.studio/nb/altinn-studio/v8/guides/development/assistant/

Acceptance criteria:

• Contains information about where data is processed, and which model provider(s) we use
• Architecture section has been removed (docs should be user facing, remove backend technicalities)
• Retention days has been updated to 90 days for threads
• Link to branching documentation
• Contains information about billing (free in beta, likely to incur costs after beta)
• Includes information about who has access (closed beta)
• Encourage users to use the feedback feature
• Encourage users to create a Github issue if they have a suggestion for improvement
• Urge users not to upload sensitive information
• Include information that we have in the AboutAssistantDialog component
• Link to docs in the AboutAssistantDialog component

Optional

• Link to skills folder, so that users can download and use them themselves

[ErlingHauan]

PR that updates documentation: Altinn/altinn-studio-docs#3011

[ErlingHauan]

We should maybe include a "how to get started with beta testing" section including feature flag activation.

Edit: Feature flag update: Altinn/altinn-studio-docs#3012

[ErlingHauan]

Link to docs from UI:
Altinn/altinn-studio#20166