Add documentation for CPU Affinity usage

[JakubPietrakIntel]

Adding a manual page in https://pytorch-geometric.readthedocs.io Advanced Topics: "CPU Affinity for PyG Workloads"
The manual explains how one can set a CPU affinity mask to improve the performance of PyG workloads.
It covers DataLoader feature enable_cpu_affinity() that can be enabled as contextmanager from AffinityMixin class, and other tools such as numactl and affinitization commands from libiomp5 and libgomp libraries.
The article provides sources to Pytorch Tutorials, where an advanced user may found more information on the methods used to maximize CPU performance with affinity masks.

[codecov]

Codecov Report

Merging #6922 (092a505) into master (06ac5a4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 092a505 differs from pull request most recent head c3e725f. Consider uploading reports for the commit c3e725f to get more accurate results

@@           Coverage Diff           @@
##           master    #6922   +/-   ##
=======================================
  Coverage   91.46%   91.46%           
=======================================
  Files         431      431           
  Lines       23457    23458    +1     
=======================================
+ Hits        21454    21455    +1     
  Misses       2003     2003           

Impacted Files	Coverage Δ
torch_geometric/loader/__init__.py	100.00% <100.00%> (ø)
torch_geometric/loader/mixin.py	87.50% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more