Documentation: Hexagonal Numbers

[sakshichahal53]

Do the checklist before filing the issue:

• [x] Do you have Node.js and all the relevant dev-dependencies installed?
• [ ] Is this a bug fix?
• [x] Is this an enhancement?
• [ ] Is this a feature request?

[fave77]

@sakshichahal53 there won't be any separate document for Hexagonal Number. It's already included in issue #180