Documentation: trace: Refactor toctree

Documentation: trace: Refactor toctree

Refactor table of contents of kernel tracing subsystem docs to improve
clarity, structure, and organization:

- Reformat sections and add appropriate headings
- Improve section grouping and refine descriptions for each group
- Add docs intro paragraph

Signed-off-by: Purva Yeshi <purvayeshi550@gmail.com>
Link: https://lore.kernel.org/r/20250318113230.24950-2-purvayeshi550@gmail.com
[Bagas: massage commit message and address reviews]
Co-developed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>