Add back hello-numpy-sag and update references

[nvkevlu]

Add back hello-numpy-sag and update references.

Description

After some thought, the hello-numpy-sag example is still relevant and is used as a base for some other examples so it is still good to keep, although I have updated the links and position in the documentation for hello-fedavg-numpy to be more prominent.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.

[chesterxgchen]

/build

[chesterxgchen]

/build