website: Fix accessibility issues to match a11y standards

[rohit-bharmal]

• Add alt attributes to images (nav event image, CNCF logos)
• Give links discernible names (navbar brand, scroll-to-overview, aria-labels)
• Improve contrast for navbar badges and adopters button
• Make links distinguishable without color (underlines, focus-visible)
• Fix heading order (section headings h3 -> h2)

Closes: #4312

[google-oss-prow]

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[google-oss-prow]

Hi @rohit-bharmal. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[rohit-bharmal]

/assign

[rohit-bharmal]

Before changes A11Y score:

After changes A11Y score:

[juliusvonkohout]

Hello, you need to sign your commits. @varodrig @thesuperzapper @tarekabouzeid for review.

[juliusvonkohout]

/ok-to-test