docs: re-organize internal image docs

[MikeMcC399]

Issue

The directories

• base-internal
• browsers-internal

contain no single informational README file, instead they contain a collection of release-specific directories. As seen in the original external image directories, keeping version-dependent sub-directories made it difficult to locate the associated README file.

Change

README files are added and structured directories are set up:

In https://github.com/cypress-io/cypress-docker-images/tree/master/base-internal:

README.md
  releases
    node-18
    ubuntu

https://github.com/cypress-io/cypress-docker-images/tree/master/browsers-internal:

README.md
  releases
    node-18

[cypress-app-bot]

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