docs: add service virtualization and sandbox testing links to concept pages

[sancharinipanda]

What has changed?

• Added "service virtualization" next to "dependency virtualization" in the intro, meta description and keywords of What is Keploy, Keploy Features and Mock Your Own Tests.
• Linked those intros to Keploy's guides on service virtualization (https://keploy.io/blog/community/service-virtualization) and sandbox testing (https://keploy.io/blog/community/sandbox-testing).
• Fixed the "PostgresSQL" typo on Supported Dependencies.

Type of change

• Documentation update (if none of the other choices apply).

How Has This Been Tested?

Text and link changes only, in versioned_docs/version-4.0.0. These changes pass the repo's Prettier 2.8.8 check. Not built locally.

Checklist:

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.

[github-actions]

Thank you and congratulations 🎉 for opening your very first pull request in keploy

[amaan-bhati]

LGTM