[tracing][doc] PromptFlow Span Specifications

[liucheng-ms]

Description

This pull request introduces a update to the PromptFlow span documentation. The doc include refined explanations of the span types, standardized formatting for attributes and events, and the addition of a conclusion section that emphasizes the importance of adhering to the tracing specifications for system transparency and consistency.

All Promptflow Contribution checklist:

• The pull request does not introduce [breaking changes].
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.
• Create an issue and link to the pull request to get dedicated review from promptflow team. Learn more: suggested workflow.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.