docs: re-organize internal image docs (#1120)

base-internal/README.md

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+# cypress/base-internal
+
+>[!NOTE]
+>`cypress/base-internal` Docker images are intended for internal Cypress.io use only. Please see [cypress/base](../base) for public images.
+
+For content details of each image, see the corresponding [releases](./releases/) sub-directory.
+
+## Tags
+
+[cypress/base-internal](https://hub.docker.com/r/cypress/base-internal/tags) images on [Cypress on Docker Hub](https://hub.docker.com/u/cypress) use image tags in the form:
+
+- `<node version>`
+- `<operating system>`-node`<node version>`
+
+for example:
+
+- `cypress/base-internal:18.20.2`
+- `cypress/base-internal:ubuntu24-node18`
+
+Images which do not contain an operating system in their tag are based on Debian.

browsers-internal/README.md

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+# cypress/browsers-internal
+
+>[!NOTE]
+>`cypress/browsers-internal` Docker images are intended for internal Cypress.io use only. Please see [cypress/browsers](../browsers) for public images.
+
+ Images are generally built from [cypress/base-internal](../base-internal/) images. For content details of each image, see the corresponding [releases](./releases/) sub-directory.
+
+## Tags
+
+[cypress/browsers-internal](https://hub.docker.com/r/cypress/browsers-internal/tags) images on [Cypress on Docker Hub](https://hub.docker.com/u/cypress) use image tags in the form:
+
+- node`<node version>`-chrome`<chrome version>`-ff`<firefox version>`-edge
+
+for example:
+
+- `cypress/browsers-internal:node18.18.2-chrome125-ff126-edge`