Drop shebang from upload_doc.sh

[cottsay]

🦟 Bug fix

Summary

This file isn't directly executable in any of its forms:

• When checked into the repository, it is a template
• When installed by gz-cmake, it is still a template
• When expanded in downstream packages, it isn't explicitly given executable permission, requiring an explicit interpreter invocation

Additionally, the usage for this script instructs the user to explicitly provide the interpreter. To avoid misleading users and linters into believing that this file should be executed directly, drop the (currently unused) shebang.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[azeey]

I think it's fine to drop the shebang. Not many people use this script directly. It's mainly used by CI to generate and upload documentation, and there, the interpreter is explicitly provided (see https://github.com/gazebosim/docs/blob/baafa500efa6b29ef2ad99f929bbba616435794d/tools/scripts/build_gz.sh#L33)