This commit updates the Overview section of the authentication documentation
to provide a clearer and more comprehensive explanation of how Tekton handles
authentication for Git and Docker.

The updated Overview section:

- Lists the supported Secret types for Git and Docker authentication
- Explains how TaskRuns and PipelineRuns access Secrets through their associated ServiceAccount and required annotations
- Clearly separates the two stages of authentication: Pod scheduling/image pulling and Step execution
- Describes the credential initialization process in a numbered list
- Specifies the files/directories generated for Git and Docker authentication within the container
- Mentions the specific rules for credential formatting and merging per Secret type

The previous Overview section lacked clarity and completeness, which could
lead to confusion for users. The updated section aims to provide a solid
foundation for understanding Tekton's authentication mechanisms before diving
into the details in the subsequent sections.

Related issue: tektoncd#7737

and also the description, "When the Steps execute, Tekton uses those credentials
to access the target Docker registry." has now been changed
Changes to be committed:
      modified:   docs/auth.md

This commit updates the "Configuring Docker Authentication" section to provide
a clearer distinction between the use cases of Docker authentication for:

1. Image Pulling Authentication: Using `dockercfg` and `dockerconfigjson`
   Secrets as `imagePullSecrets` to authenticate with Docker registries and
   pull container images during the Pod creation phase.

2. In-Pod Docker Authentication: Setting up Docker authentication within the
   Pod's container environment by generating a `~/.docker/config.json` file
   after the Pod is created and images are pulled. This authentication setup
   allows the Steps within the Pod to interact with Docker registries during
   execution.

The section explains that the Docker authentication credentials used for
in-Pod authentication are derived from the same Secrets specified as
`imagePullSecrets`, and Tekton follows the credential formatting and merging
rules defined by the Secret types.

The commit also maintains the existing examples for defining the Secret and
associating it with the ServiceAccount and Run.

Resolves tektoncd#7737