Link to Adam's staging environment article from the glossary

[codedbychavez]

Description

I've added Adam's article "What is a staging environment" to the blog with PR: https://github.com/configcat/blog/pull/321

This PR adds the term Staging Environment to the glossary and with a back-link to the above article.

IMPORTANT: MERGE THIS AFTER https://github.com/configcat/blog/pull/321 IS MERGED AND IS LIVE.

Trello card

link to the Trello card

Notes for QA

1. Which part of the application was affected by the changes?

• The term "Staging Environment" was added to the website/docs/glossary.md file.

2. What needs testing?

• Verify if clicking on the "Staging Environment" term redirects you to the blog article mentioned in the description section above.
• This will only work once the above article in merged and is live.

Requirement checklist

• I have validated my changes on a test/local environment.
• I have checked the SNYK/Dependabot reports and applied the suggested changes.
• (Optional) I have updated outdated packages.

[sigewuzhere]

Moved to another PR: #328