doxygen: improve doc for kernel basics section

Organize the document content of the "Kerenl Basics" section.
This PR does not make substantial changes to the text content,
but only makes some optimizations and adjustments to the
document formatting, including:

- Enclose the function name and variable name in "`", so that
  the HTML rendering will use different fonts to distinguish
  them from the text part.

- Add blank lines between paragraphs, otherwise the HTML
  rendering will merge the paragraphs without blank lines
  into one paragraph.

- Use "@ref" to reference the page of the corresponding
  chapter, such as the reference to page_thread_management
  at the end of the "Thread Scheduling" chapter.

- For tables in markdown, try to align the columns in the
  text to beautify the reading experience.

Signed-off-by: Chen Wang <[email protected]>

Author: unicornx