Add rel=noopener to social icon and share button anchors

[gavinwray]

Description

Open external anchors for the social icons and share buttons using rel="noopener".

Motivation and Context

Good practice for security and performance, as recommended in Google's Tools for Web Developers.

How Has This Been Tested?

Hugo Version:
v0.42.1

Browser(s):

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation, including theme.toml, accordingly.
• I have read the Contributing Document.