Update the contributing guide about pushing changes to dvc and git

[seisman]

Description of proposed changes

1. Run dvc status --remote upstream before dvc push
2. Run dvc push before git push

Fixes #1765

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[maxrjones]

Should we remove the following line since all the tests are now managed with dvc?

git rm -r --cached 'pygmt/tests/baseline/test_logo.png'  # only run if migrating existing image from git to dvc

[maxrjones]

I think some explanatory comments could be helpful.

[seisman]

Should we remove the following line since all the tests are now managed with dvc?

git rm -r --cached 'pygmt/tests/baseline/test_logo.png'  # only run if migrating existing image from git to dvc

Done in e8a3ad6

[weiji14]

Just one minor suggestion, otherwise looks good!