Combine content for "contributing code" concepts

[afred]

Pages affected

1. site_content/_pages/contributing_code/code.md
2. site_content/_pages/contributing_code/coding_style.md
3. site_content/_pages/contributing_code/how-to-pr.md
4. site_content/_pages/contributing_code/issues.md
5. site_content/_pages/contributing_code/pr_checklist.md
6. site_content/_pages/contributing_code/review.md

Describe the changes needed

The page code.md seems to have a lot of overlap with the other pages. I think these were maybe intended to compliment each other at some point, because code.md is linked to from other pages.

But the effect is that code.md says a little but about all of the concepts, while all the other pages cover the same concepts, only slightly differently. This is not as useful as having all the content for a given concept in a one location.

Done when

All the content for each section within code.md has been merged with content from the corresponding page that also covers the same concept.