Doc has inconsistent use of LlmAgent and Agent even in the same page

Problem:

- The code examples in the documentation create a confusing experience by alternating between LlmAgent and Agent for initialization. A user might follow one example, and then in the next section, the required import changes without any context or explanation for the switch.

Proposed Solution:

- For better clarity and consistency, the documentation should be standardized to use a single class, such as LlmAgent, across all examples. An explicit note should be added to inform users that LlmAgent and Agent are functionally identical and can be used interchangeably.

While [a note exists](https://google.github.io/adk-docs/agents/llm-agents/) stating they are the same, its current wording is ambiguous. I initially interpreted it to mean LlmAgent is simply referred to as Agent conceptually, not that they were equivalent imports.