Fixed width for current page section button on Sidebar

[lamATnginx]

Proposed changes

Closes https://github.com/nginxinc/docs-platform/issues/580

Before (Firefox):

After:

Verified it doesn't break as well in Chrome and Safari

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[github-actions]

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[lamATnginx]

justify-self doesn't work with flex.

From their docs - "In flexbox layouts, this property is ignored. Read more about alignment in flexbox."

[danielledeleo]

Thanks!