DOC-2597: Provide demo for Image Optimizer Signed Uploads option

[FarzadHayat]

Ticket: DOC-2597

Site: uploadcare_signed_upload_auth_provider
Site: uploadcare_store_type

Changes:

• DOC-2597: Add link to official Uploadcare documentation for Signed Uploads
• DOC-2597: Add admonition with link to Signed Uploads starter project
• DOC-2597: Update description for store type to be more accurate

Pre-checks:

• Branch prefixed with feature/<version>/, hotfix/<version>/, staging/<version>/, or release/<version>/.
• [-] modules/ROOT/nav.adoc has been updated (if applicable).
• [-] Included a release note entry for any New product features.
• [-] If this is a minor release, updated productminorversion in antora.yml and added new supported versions entry in modules/ROOT/partials/misc/supported-versions.adoc.

Review:

• Documentation Team Lead has reviewed