feat(identity): reference for the access tokens duration

[juliamrch]

Description

This PR updates the existing Identity docs to show users how to customize the duration (TTL) for access tokens issued by Kong Identity clients. There is a distinction between clients that users create using the API and clients dynamically created through the dev portal or other automations. They set the duration accordingly.

Users can customize this in 2 ways:

• Using the API
• Using Terraform

closes #6690

Things to check before merging:

Are the Terraform Konnect provider links accurate?

• https://github.com/Kong/terraform-provider-konnect/blob/main/examples/resources/identity_auth_server.tf)
• https://github.com/Kong/terraform-provider-konnect/blob/main/examples/resources/konnect_identity_auth_server_client.tf

Preview Links

• https://deploy-preview-6751--kongdeveloper.netlify.app/how-to/kong-identity-dcr/#create-an-auth-server-in-kong-identity
• https://deploy-preview-6751--kongdeveloper.netlify.app/dev-portal/dynamic-client-registration/#authentication-methods
• https://deploy-preview-6751--kongdeveloper.netlify.app/identity/auth-servers/#access-token-duration
• https://deploy-preview-6751--kongdeveloper.netlify.app/identity/auth-servers/#create-an-auth-server-claim-and-client-in-kong-identity

Checklist

• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable).

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