docs: update `CONTRIBUTING.md`

[angeloashmore]

Resolves: N/A

Description

This PR replaces the existing CONTRIBUTING.md with a new, simplified version.

Checklist

• [ ] If my changes require tests, I added them.
• [ ] If my changes affect backward compatibility, it has been discussed.
• [x] If my changes require an update to the CONTRIBUTING.md guide, I updated it.

Preview

• https://github.com/prismicio/prismic-client/blob/aa/docs/CONTRIBUTING.md

How to QA [^1]

Read through CONTRIBUTING.md and let me know what you think.

[^1]: Please use these labels when submitting a review: :question: #ask: Ask a question. :bulb: #idea: Suggest an idea. :warning: #issue: Strongly suggest a change. :tada: #nice: Share a complement.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.97%. Comparing base (f141026) to head (fb57311). Report is 2 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #340 +/- ## ======================================= Coverage 99.97% 99.97% ======================================= Files 61 61 Lines 6899 6911 +12 Branches 381 381 ======================================= + Hits 6897 6909 +12 Misses 2 2 ```

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[angeloashmore]

The typos were fixed and PULL_REQUEST_TEMPLATE.md was removed in favor of the org-level version. We should be good to merge!