Fix transparent notification dropdown menu on mobile

[josh-wong]

Description

This PR fixes an issue where the notification dropdown menu (introduced in #121) is transparent when expanded on mobile devices.

Related issues and/or PRs

• Add in-site notification feature #121

Changes made

• Changed the position style for .notification-dropdown to make the dropdown menu cover the icons behind the dropdown menu on mobile.
• Changed the width style for .navbar-sidebar .notification-wrapper to make the notification dropdown menu stay within the expanded menu on mobile.
• Changed the position style for .header-rss-link::before to make the RSS icon match the same behavior of the other icons in the navbar on mobile.
• Reduced the distance between the notification bell icon and the notification count.

Checklist

The following is a best-effort checklist. If any items in this checklist aren't applicable to this PR, add N/A after each item.

Documentation

• I have updated the side navigation as necessary. N/A
• I have updated the documentation to reflect the changes. N/A
• I have documented or updated any remaining open issues linked to this PR in GitHub, Obsidian, etc.

Build, deploy, and test

• I have merged and published any dependent changes in other PRs. N/A
• I have commented my code, particularly in hard-to-understand areas. N/A
• I have checked that my changes look as expected on a locally built version of the docs site.
• My changes generate no new warnings.

[github-actions]

All items in the checklist have been checked🎉

[github-actions]

All items in the checklist have been checked🎉

[github-actions]

All items in the checklist have been checked🎉