docs: Fix typos, broken links, and grammar across documentation

Merge https://github.com/google/adk-python/pull/3937

**Please ensure you have read the [contribution guide](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) before creating a pull request.**

### Link to Issue or Description of Change
Not applicable

**Problem:**
Several markdown files contained typos, grammatical errors (e.g., "search youtubes"), and awkward phrasing.
**Solution:**
Performed a comprehensive quality assurance pass on the documentation.
- Fixed typos in README.md and AGENTS.md.
- Improved grammar and phrasing in CONTRIBUTING.md and sample READMEs.

### Testing Plan

This is a documentation and typo fix PR.

**Unit Tests:**

- [ ] I have added or updated unit tests for my change.
- [ ] All unit tests pass locally.
N/A - Documentation changes only.

**Manual End-to-End (E2E) Tests:**

This is a documentation and typo fix PR.

### Checklist

- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [ ] I have commented my code, particularly in hard-to-understand areas.
- [ ] I have added tests that prove my fix is effective or that my feature works.
- [ ] New and existing unit tests pass locally with my changes.
- [x] I have manually tested my changes end-to-end.
- [ ] Any dependent changes have been merged and published in downstream modules.

Co-authored-by: Xiang (Sean) Zhou <seanzhougoogle@google.com>
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/3937 from Goodnight77:docs/fix-typos a0cf4db6741f19c77eeb0746c9db524dd02121ac
PiperOrigin-RevId: 845599254

contributing/samples/live_bidi_streaming_single_agent/readme.md

@@ -8,7 +8,7 @@ Follow these steps to get the agent up and running:
 
 1.  **Start the ADK Web Server**
     Open your terminal, navigate to the root directory that contains the 
-    `live_bidi_streaming_agent` folder, and execute the following command:
+    `live_bidi_streaming_single_agent` folder, and execute the following command:
     ```bash
     adk web
     ```