refactor(pull-request-guidelines): organize contents

[kenji-miyake]

Description

• 820d1b724112fdbfcd822f3ff22538a8ac4dc094: Split the CI checks page.
• ff2ea55333f24fe2b74c9d7a0f4fcf9d04cdb64e: Move the pull request rules from the commit guidelines.

Related: https://github.com/autowarefoundation/autoware-documentation/pull/194

Pre-review checklist for the PR author

The PR author must check the checkboxes below when creating the PR.

• [x] I've confirmed the contribution guidelines.
• [x] The PR follows the pull request guidelines.

In-review checklist for the PR reviewers

The Reviewers must check the checkboxes below before approval.

• [x] The PR follows the pull request guidelines.

Post-review checklist for the PR author

The PR author must check the checkboxes below before merging.

• [ ] There are no open discussions or they are tracked via tickets.

After all checkboxes are checked, anyone who has write access can merge the PR.

[kenji-miyake]

pre-commit-optional will be fixed in https://github.com/autowarefoundation/autoware-documentation/pull/194.

[kenji-miyake]

The content is approved in https://github.com/autowarefoundation/autoware-documentation/pull/194.