Aria-label improvements for links that open in a new tab.

[CambodianCoder]

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

Links that open in a new tab don't respect the existing label field specified in the docusaurus config file.
Please refer to #11331 (comment) for more details.

Reproducible demo

No response

Steps to reproduce

Create a link in the docusaurus config file that doesn't have a "to" field, indicating it should open in a new tab.

Expected behavior

The supplied label is used, along with the text "(Opens in a new tab.)", in the aria-label.

Actual behavior

The supplied label was not used in the aria-label.

Your environment

• Public source code:
• Public site URL:
• Docusaurus version used:
• Environment name and version (e.g. Chrome 89, Node.js 16.4):
• Operating system and version (e.g. Ubuntu 20.04.2 LTS):

Self-service

• I'd be willing to fix this bug myself.