"Introduction to Cosign" and "How to Install Cosign" Sorted to the Bottom

[mohammed90]

Describe the bug
The "Introduction to Cosign" and "How to Install Cosign" navigation links are at the bottom of the list under the "Cosign" header.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://edu.chainguard.dev/open-source/sigstore/
2. Scroll to the bottom
3. Click on "Cosign"
4. See the navigation list
5. Click on any of the items
6. See the navigation list under "Cosign" on the left

Expected behavior
The "Introduction to Cosign" and "How to Install Cosign" navigation links are at the top of the list under the "Cosign" header.

Screenshots

Desktop (please complete the following information):

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Smartphone (please complete the following information):

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Additional context
The fix is to adjust the weight in the front-matter of those 2 pages to be smaller than the rest. I just want to make sure you're all on board with this change and reasoning.

[ltagliaferri]

Thanks @mohammed90 — I agree with the change! I made some minor other reorg changes, too. I can't tag you as a reviewer, but let me know if you have any other thoughts on #138