Add "Memory consistency models" subsection to "Memory orderings" section

[yutingshih]

This pull request aims to enhance the comprehensiveness of the “Memory orderings” section of this book. Currently, the section primarily covers memory orders defined in C11/C++11 but lacks discussion on hardware assumptions regarding memory consistency. Thus I made the following changes:

1. Reorganization
   • Moved the content originally under “Memory orderings” related to C11/C++11 atomics to a dedicated subsection titled “C11/C++ atomics” within the “Memory orderings” section.
2. Addition
   • Introduced a new subsection titled “Memory consistency models”
     • Detailed descriptions of sequential consistency, x86 total store order, and ARM’s relaxed memory order.
     • Included schematic diagrams illustrating hardware architectures and litmus tests to aid readers’ comprehension.

[jserv]

Any progress?