Add num_perfect_squares #767

[mantaur]

The original implementation by @ntomsic for #767 with an open PR wanted these changes:

• https://github.com/keon/algorithms/pull/769#discussion_r619754281
• https://github.com/keon/algorithms/pull/769#discussion_r619754890

But since the requested changes from 25/04/2021 haven't been fixed yet, I assumed @ntomsic has dropped this. Another pr for #767 was opened: https://github.com/keon/algorithms/pull/793 , however @rashidkalwar didn't implement the requested changes. This pr fixes #767 the requested changes and improves documentation.

[ankit167]

Please resolve conflicts

[mantaur]

@ankit167 Done!