DOC: Update URLs from "http" to "https" if available

[yvonnefroehlich]

Description of proposed changes

This PR updates some URLs from "http" to "https".

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.
• Use underscores (not hyphens) in names of Python files and directories.

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]

Good catches. Could you also please do:

1. in .github/ISSUE_TEMPLATE/1-bug_report.yaml, change http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports to https://matthewrocklin.com/minimal-bug-reports
2. in doc/_templates/footer.html, update http://sphinx-doc.org/