Fix #509, fixed all broken links for this issue

[shourysingh07]

Fixed the web accessibility contribution documentation link and contributor guidelines link in Contributing to the p5.js Reference.

Before

before.mp4

After

after.mp4

[Qianqianye]

Thanks @shourysingh07 for working 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.

The broken links mentioned in this PR are fixed in this commit and this commit within p5.js library repo. The updates are now reflected on the contributing_to_the_p5js_reference.mdx file and Contributing to the p5.js Reference web page.

In the future, if you find more broken links of contribute pages, please feel free to go ahead and open PRs within p5.js library repo. I will close this PR for organizational purpose. Thank you!