POC: Add PyGMT logo to README & docs

[michaelgrund]

Description of proposed changes

Since we now have a logo (#1404) we should think about in which format we want to place it in our README/docs. I would suggest to use the horizontal one. Would also suggest to add the subtitle A Python interface for the Generic Mapping Tools. Maybe @sfrooti may add that to the prepared logos. Also need to increase the resolution. Finally, the yellow lines do not really stand out from the background.

Preview:

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 command is:

• /format: automatically format and lint the code

[seisman]

Thanks for opening the PR so that we can better preview what the logo looks like on the web or PDF. Just want to note that the logo is not finalized yet, and we may tweak the logo based on the community responses.

[seisman]

@michaelgrund Please help replace the favicon doc/_static/favicon.png, and also maybe add a logo to the title at the sidebar by setting html_logo to an image path in doc/conf.py.