doc: Make it clearer what the auth documentation is about #7737

[leodahal4]

Changes

This PR addresses the issues raised in #7737 regarding the clarity and accuracy of the authentication documentation, specifically the sections related to Docker authentication.

Key Changes:

Overview Section:

	Provides a comprehensive explanation of the authentication process, including the two stages (Pod Scheduling and Image Pulling, and Step Execution).
	Describes the credential initialization process and the generated credential file locations.

	Configuring Docker Authentication Section:

	Clarifies the role of imagePullSecrets and their usage during the Pod creation phase for pulling images from private registries.
	Includes examples of specifying imagePullSecrets in run.yaml for TaskRun and PipelineRun.

Benefits:

	Addresses confusion and lack of clarity in the existing documentation.
	Improves understanding of when and how Docker credentials are used, particularly during the Pod creation phase.
	Enhances comprehension of the authentication flow and setup within the container.

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• [x] Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• [ ] Has Tests included if any functionality added or changed
• [ ] pre-commit Passed
• [ ] Follows the commit message standard
• [ ] Meets the Tekton contributor standards (including functionality, content, code)
• [ ] Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• [ ] Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings). See some examples of good release notes.
• [ ] Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

NONE

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: leodahal4 / name: Mou Sam Dahal (d944b4c644eab06c5754655ea3f9c400632c92b0, 7d5b243b8d1719e708c6de815fb910e05b9be5b2, d531f2f6ea0f19beb80ea2569fe1c4ceeed2975e, 4e95f12ef7309aae89e42a7b7c28496408376c41, c37f8141398fb5bf9c5d9dffacf9428f55e7fea0, 756bdf1ac84112fcb8dddd991ccff2133d61ecc6)

[tekton-robot]

Hi @leodahal4. Thanks for your PR.

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[khrm]

/ok-to-test

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: chitrangpatel

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[leodahal4]

@vdemeester can you review this PR again ? I have made some changes for clarification on both of the things.

[afrittoli]

@leodahal4 thank you for your contribution! Are you still working on this PR?

[vdemeester]

/hold

[vdemeester]

@leodahal4 can you rebase instead of merge from main ? as is, it can't be merge by prow/the bot