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/adk_project_overview_and_architecture.md

@@ -34,7 +34,7 @@ my_adk_project/
     └── my_app/
         ├── agents/
         │   ├── my_agent/
-        │   │   ├── __init__.py   # Must contain: from. import agent \
+        │   │   ├── __init__.py   # Must contain: from . import agent \
         │   │   └── agent.py      # Must contain: root_agent = Agent(...) \
         │   └── another_agent/
         │       ├── __init__.py
@@ -43,7 +43,7 @@ my_adk_project/
 
 agent.py: Must define the agent and assign it to a variable named root_agent. This is how ADK's tools find it.
 
-`__init__.py`: In each agent directory, it must contain from. import agent to make the agent discoverable.
+`__init__.py`: In each agent directory, it must contain `from . import agent` to make the agent discoverable.
 
 ## Local Development & Debugging
 
@@ -69,7 +69,7 @@ We expose agents as production APIs using FastAPI.
 
 - Custom Endpoints: We can add our own routes (e.g., /health) to the app object returned by the helper.
 
-Python
+```Python
 
 from google.adk.cli.fast_api import get_fast_api_app
 app = get_fast_api_app(agent_dir="./agents")
@@ -77,6 +77,7 @@ app = get_fast_api_app(agent_dir="./agents")
 @app.get("/health")
 async def health_check():
     return {"status": "ok"}
+```
 
 
 ## Deployment to Production