Add MASTG-TECH-0142 for inspecting Android WebView storage

[SuyashJain17]

This PR closes #3602

Description

Adds MASTG-TECH-0142 to document direct inspection of Android WebView
storage under /data/data/<app_package>/app_webview/ using adb,
@MASTG-TOOL-0006, and Android Studio.

Note: The issue references MASTG-TEST-0320 as an example. This test does not
currently exist, and existing MASVS-PLATFORM tests were reviewed without
finding a suitable location for a reference. This change therefore
focuses on adding the missing technique.

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Contributor Checklist

• I have read and understood the contributing guidelines.
• I followed the project style guide.
• I validated the technical correctness of my changes and understand the topic.
• This PR adds clear value and is not spam or low-effort content.

Relevant documentation.

• General Contributing Guidelines
• Style Guide
• Porting MASTG v1 Tests to v2
• Instructions for new MASTG components

Contributors are expected to understand basic git and GitHub workflows, including forks, branches, commits, and pull requests. The project does not provide training. Pull requests that do not meet these minimum requirements may be closed without review.

[cpholguera]

Please also update the declarations in the PR description: #3605

[cpholguera]

• Please resolve the conflicts, the goal was to update the existing TECH, not to create a new one.
• The mentioned test does exist and should be accordingly updated: https://mas.owasp.org/MASTG/tests/android/MASVS-PLATFORM/MASTG-TEST-0320/
• Please do not break lines as you're doing. Each paragraph should be a line.
• No need to justify why the technique is needed or what the best practices are.
• Use @MASTG-TOOL-XXXX IDs everywhere.
• If another MASTG-TECH covers part of the steps here you should use it instead (e.g. for "Obtain a shell").
• Refer to MASTG-KNOW-0018 for "what to expect within this folder".