Update Overview documentation (#912)

- Added information about functional agents.
- JetBrains is now explicitly mentioned in the framework description.

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## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->

## Breaking Changes
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#### Type of the changes
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [x] Documentation update
- [ ] Tests improvement
- [ ] Refactoring

#### Checklist
- [ ] The pull request has a description of the proposed change
- [ ] I read the [Contributing
Guidelines](https://github.com/JetBrains/koog/blob/main/CONTRIBUTING.md)
before opening the pull request
- [ ] The pull request uses **`develop`** as the base branch
- [ ] Tests for the changes have been added
- [ ] All new and existing tests passed

##### Additional steps for pull requests adding a new feature
- [ ] An issue describing the proposed change exists
- [ ] The pull request includes a link to the issue
- [ ] The change was discussed and approved in the issue
- [ ] Docs have been added / updated

Author: innateteniuk

docs/docs/index.md

@@ -1,12 +1,13 @@
 # Overview
 
-Koog is a Kotlin-based framework designed to build and run AI agents entirely in idiomatic Kotlin.
+Koog is an open-source JetBrains framework designed to build and run AI agents entirely in idiomatic Kotlin.
 It lets you create agents that can interact with tools, handle complex workflows, and communicate with users.
 
 The framework supports the following types of agents:
 
 * Single-run agents with minimal configuration that process a single input and provide a response.
   An agent of this type operates within a single cycle of tool-calling to complete its task and provide a response.
+* Functional agents with lightweight, customizable logic defined by a lambda function to handle user input, interact with an LLM, call tools, and produce a final output.
 * Complex workflow agents with advanced capabilities that support custom strategies and configurations.
 
 ## Key features