[Fundamentals] Concepts cleanup

[dcpena]

• Removed What is Cloudflare and Internet pages.
• Added redirects for pages above
• Moved Our Free Plan to Billing and Subscriptions and added redirect

Addresses PCX-15745

[github-actions]

Howdy and thanks for contributing to our repo. We review internal PRs within 1 week. If it's something urgent or has been sitting without a comment, start a thread in the Developer Docs space internally.

[github-actions]

This PR changes current filenames or deletes current files. Make sure you have redirects set up to cover the following paths:

• /fundamentals/concepts/the-internet/
• /fundamentals/concepts/what-is-cloudflare/
• /fundamentals/concepts/reference-architecture/
• /fundamentals/concepts/free-plan/

[github-actions]

Preview URL: https://1827af4f.preview.developers.cloudflare.com
Preview Branch URL: https://dpena-concepts-cleanup.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/fundamentals/concepts/what-is-cloudflare/	https://dpena-concepts-cleanup.preview.developers.cloudflare.com/fundamentals/concepts/what-is-cloudflare/
https://developers.cloudflare.com/fundamentals/concepts/the-internet/	https://dpena-concepts-cleanup.preview.developers.cloudflare.com/fundamentals/concepts/the-internet/
https://developers.cloudflare.com/fundamentals/subscriptions-and-billing/free-plan/	https://dpena-concepts-cleanup.preview.developers.cloudflare.com/fundamentals/subscriptions-and-billing/free-plan/
https://developers.cloudflare.com/learning-paths/get-started/concepts/how-cloudflare-works/	https://dpena-concepts-cleanup.preview.developers.cloudflare.com/learning-paths/get-started/concepts/how-cloudflare-works/
https://developers.cloudflare.com/fundamentals/reference-architecture/	https://dpena-concepts-cleanup.preview.developers.cloudflare.com/fundamentals/reference-architecture/