CI: Run certain GitHub Actions workflows on official repo only

[weiji14]

Description of proposed changes

Noticed at https://github.com/seisman/pygmt/pull/2 that there were a lot of GitHub Actions workflows being ran that might not make sense for forks, so disabling some of them. Specifically:

• benchmarks.yml
• check-links.yml
• ci_docs.yml
• ci_doctests.yaml
• ci_tests_legacy.yaml
• release_drafter.yml

The list above are mostly those that run on the main branch or on a cron schedule, which GitHub should have disabled by default already for new forks according to https://github.com/orgs/community/discussions/26704#discussioncomment-5268498. But we can still explicitly disable them.

The benchmarks.yml one should be disabled, otherwise it will error with a message like this (copied from https://github.com/seisman/pygmt/actions/runs/7403301366/job/20143268291?pr=2):

  Preparing upload...
  Error: error decoding response body: missing field `status` at line 1 column 58
  Error: Process completed with exit code 1.

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 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 commands are:

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

[seisman]

Makes sense to me.