MINOR: Fix numbered list formatting in security documentation (#21324)

The numbered lists in the security documentation were indented by 2–8
spaces, causing Markdown to treat them as plain text rather than lists.
This resulted in the list items being rendered without proper line
breaks and collapsed into a single paragraph.

### Changes:
  - Removed excessive indentation from numbered list items (now start at
column 0)
  - Added proper blank lines between list items for better readability
  - Fixed bullet list indentation to properly nest under numbered items
(using 3 spaces)
  - Added code fences (```) around code blocks for clearer formatting

Reviewers: Chia-Ping Tsai <chia7712@gmail.com>

Author: mingyen066