docs: 📘 Added option to add functions to multiple categories

[theryansmee]

Add multi category logic. Update functions with to have multiple
categories

✅ Closes: #175

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/falso/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: 175

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information