add dependabot for GHA and linkchecker

[ocefpaf]

Check for dead links and update the GitHub Actions. Should help catch issues like #41 early.

[ocefpaf]

I believe I fixed or found replacements for most broken links except for:

• http://mmisw.org/orr/#http://mmisw.org/ont/ioos/sector → Status: 404
• https://mmisw.org/orr/#http://mmisw.org/ont/ioos/platform → Status: 404
• http://www.wmo.int/pages/prog/www/TEM/GTS/index_en.html → Status: 404
• https://gliders.ioos.us/index.html → Status: 404

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Edit: in https://github.com/ioos/glider-dac/pull/259/files we used,

• http://www.wmo.int/pages/prog/www/TEM/GTS/index_en.html →https://community.wmo.int/en/activity-areas/global-telecommunication-system-gts
• https://gliders.ioos.us/index.html → https://gliders.ioos.us/

Implementing those changes here but we still don't have replacements for the first two.

[MathewBiddle]

mmi links should go to https://mmisw.org/ont#/org/ioos

[ocefpaf]

mmi links should go to https://mmisw.org/ont#/org/ioos

Thanks @MathewBiddle!. All links are fixed and the PR is passing now!