Merge Dapr Agent Documentation into Dapr Docs

[bibryam]

Context
The current Dapr Agent documentation is difficult to discover. Consolidating this documentation into the main Dapr documentation improves visibility for developers.

Acceptance Criteria

• Dapr Agent documentation is moved into the main Dapr documentation repository.
• Content is restructured to align with the Dapr documentation style and navigation.
• Missing documentation sections are identified and added.
• Old or redundant Dapr Agent documentation pages are removed.
• All updates are completed and merged before the Dapr 1.16 release.

[bibryam]

Discussed with @marcduiker and agreed on the following

• Dapr agent docs are relocated under the "Developing Applications" section of the Dapr documentation.
• Documentation is organized in a flat menu structure as originally defined in the Dapr agent docs.
• The "Core Concepts" section becomes a menu item with individual concepts listed as sub-items.
• The outdated "Quickstarts" documentation is removed, instead a link is added pointing users to the README in the Quickstarts GitHub repository.

[bibryam]

@marcduiker can you transfer this issue to dapr docs repo? I think that would be a better place

[marcduiker]

dapr/docs#4741

[sicoyle]

This is completed

[bibryam]

follow up to this issue #180