doc: improvements to Contribution Guidelines

Restructure the Contribution Guidelines page in the documentation.

Create the 'Pull Request Guidelines' section containing the best
practices for creating Pull Requests.

Improve commit message guidelines: instruct contributors to refer to
GitHub issue in the PR message instead of commit message when
submitting a fix; add a description of `Link:` tags for refering to
other relevant information.

Move 'clang-format' and 'twister' subsections to the new PR
Guidelines section.

Delete outdated/incorrect information about how to run tests locally
with twister.

Describe the `check_compliance.py` script as the recommended tool for
checking compliance of changes and how to run the script locally.
Remove 'gitlint' subsection.

Add a list of recommended steps for getting a new PR accepted quickly.

Signed-off-by: Gustavo Silva <gustavograzs@gmail.com>