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.

[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

[gikf]

@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

Last complain then 😛. Specifically for Python, the https://contribute.freecodecamp.org/curriculum-help/#ast-based-helpers is a general overview how to use the ast_helpers, while the https://opensource.freecodecamp.org/curriculum-helpers/python.html is more a reference of all methods. Both have their place in documentation, but joining them together, on the same page, can make using it for these two separate reasons, a bit harder.

[ilenia-magoni]

@gikf the documentation can be organized to have two sections then, one with the general overview, and the other with the list of all methods, both in this repo