Fix broken links on contributor page readme

[sambensim]

• Fixed 6 links as outlined in Issue #444.

• Additionally fixed a typo in the second to last paragraph (changed from "there are many more ways to contribute to p5.js through non-source code contribution than can be exhaustively listed here" to "there are many more ways to contribute to p5.js through non-source code contribution than cannot be exhaustively listed here").

[sambensim]

@all-contributors please add @sambensim for 📖 doc

[SableRaf]

@sambensim would you mind revising this PR to focus only on the fixes to the broken links?

Feel free to open a separate issue for the copy. Though it is not a typo, the text could be made clearer (see my inline comment).

[sambensim]

@sambensim would you mind revising this PR to focus only on the fixes to the broken links?

Feel free to open a separate issue for the copy. Though it is not a typo, the text could be made clearer (see my inline comment).

Hey! Sorry just saw this, should I use the text you suggested earlier or just focus on the link fixes?

[SableRaf]

@sambensim No worries! Thanks for your reply :) I'd suggest focusing on the links.

If you want to address the text please open a separate issue. It is correct as is but your intuition to correct it shows that the phrasing could be clearer.

[Qianqianye]

Thanks @sambensim for all your work on this. The contribute pages on p5js.org website are built directly from p5.js library repo contributor_docs folder. So if we want to make changes, we will edit the docs in p5.js library repo contributor_docs folder,

All the broken links mentioned in this PR is fixed in this commit in p5.js library repo, and then this commit in p5.js website repo. In the future, if you find more broken links of contribute pages, please go ahead and open PRs within p5.js library repo.

I will close this PR for organizational purpose. Thank you!