docs: add helper documentation

[a2937]

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

Closes #539 and Closes #540

I've added documentation for all of our methods that was named on the issues. Additionally I added all of the AST Helper methods onto the curriculum helper page.

[ilenia-magoni]

with what code?

[a2937]

Does this help?

[gikf]

Is this supposed to completely replace the documentation from curriculum-helpers - https://opensource.freecodecamp.org/curriculum-helpers/ / https://github.com/freeCodeCamp/curriculum-helpers/tree/main/docs? Keeping the two copies updated is not going to be very convenient.

[ilenia-magoni]

@gikf the final goal is to have /contribute replace all the small repo docs we have around, it makes sense that also curriculum-helpers docs is folded here