Specify header sizes in style.css and remove bold formatting from contributing.md headers

[maxrjones]

Description of proposed changes
This PR specifies the header styles in style.css based on the comments in #1339 (comment) and #1335 (review) that header 4 levels are too small. The header sizes decrease linearly from 200% for header 1.

For explanation, in #1335 I suggested that this was a MyST issue, but in testing a new issue for conversion from .md to .rst I discovered that the ReST versus Markdown format did not affect the header appearance, hence the fix in style.css and the removal of the temporary fix in bolding the headers in contributing.md.

Fixes #

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 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

[weiji14]

Looks much better! I'd wait for a second (or second) opinion/approval before merging directly, preferably someone on a different browser (mine is Firefox).

[core-man]

Looks good on Chrome on a 22-inch display and Safari on a 13 inch one~

[willschlitzer]

Looks good to me (Ubuntu, Firefox, 21 inch monitor)