Revisit ordering of sections in CONTRIBUTING

[trivikr]

• I'd be willing to implement this feature (contributing guide)
• This feature is important to have in this repository; a contrib plugin wouldn't do

Describe the user story

A question was raised on whether reviewing PRs should be before or after documentation section. Creating this issue to come up with as consensus and create a PR accordingly.
Refs: #4065 (comment)

Describe the solution you'd like

Revisit the ordering of the items in CONTRIBUTING section, and reorder them based on what benefits the most to yarn community
contributors.

Describe the drawbacks of your solution

N/A

Describe alternatives you've considered

N/A

[trivikr]

Existing ordering in https://github.com/yarnpkg/berry/blob/master/packages/gatsby/content/advanced/contributing.md

• Opening an issue
• How can you help?
• Finding work to do
• Writing your feature
• Testing your code
• Formatting your code
• Checking Constraints
• Preparing your PR to be released
• Reviewing other PRs
• Writing documentation
• Profiling

[trivikr]

All the section are at the same level of heading.

How about splitting them into multiple subheadings, for example:

• Opening an issue
• How can you help?
  • Finding work to do
• Writing your feature
  • Building
  • Testing
  • Formatting
  • Checking Constraints
• Writing documentation
• Pull Requests
  • Preparing your PR to be released
  • Reviewing other PRs
• Profiling

Writing docs and profiling can be moved to under feature. I just kept them as separate section as:

• Documentation is written in the website package, and I assume it won't be applicable for most PRs. The documentation inside, if any, should be in code.
• Profiling is also not done for most PRs.