Fix ref links in contributing guide

[mabelzhang]

🦟 Bug fix

Fixes broken ref links on contributing webpage.

Summary

I just want to be able to click on things. Feel free to decline this PR if this isn't the best way to fix the links. It was minimal effort.

On the webpage, none of the internal links work https://ignitionrobotics.org/docs/all/contributing
(They only work on the GitHub page, which is the page after people click on the "edit" button on the page on the Ignition website.)

Since we're using the contributing guide more and more (including linking from the template from which this PR was created), I figure we should fix it on the website, even if that means the links on the GitHub markdown page will also link to the website?

I don't know if there's a way to make it work for both. If there is, that would be preferred.

P.S. On the page to create a PR, the template links to the contributing page on ignitionrobotics.org, but the little dialog on the right that says "Contributing guidelines" still links to the GitHub page https://github.com/ignitionrobotics/docs/blob/d56955a8ee3b19e9a1fb347c14de094a09387a8e/contributing.md
Might want to make them the same, for consistency, especially if we merge this PR.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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

[chapulina]

I don't know if there's a way to make it work for both. If there is, that would be preferred.

Agreed, we need to look into the site's source code to figure out what's going on.

the little dialog on the right that says "Contributing guidelines" still links to the GitHub page

I think GitHub is just linking to this repository's contributing.md file, which can be misleading. That file is about contributing code to Ignition, but when opening a PR to this repository, a user probably wants to know how to contribute documentation. I started #148 to discuss this.

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I think this is a good solution until we figure out the "proper" way of doing this 👍