chore: small storybook and docs updates to SensitiveText component

[georgewrmarshall]

Description

This PR updates the Storybook stories for the SensitiveText component to align with extension component library standards. The changes include:

• Updating Storybook stories to use the CSF 3.0 format for better organization and future-proofing.
• Converting README.md to README.mdx, leveraging Storybook stories for documentation and improving the accessibility and usability of the component documentation.

Related issues

Related: #28021

Manual testing steps

1. Run Storybook locally and navigate to the SensitiveText component.
2. Verify that the stories render correctly in the CSF 3.0 format.
3. Check that the new README.mdx renders properly in the Storybook Docs tab.

Screenshots/Recordings

Before

before.mov

After

after.mov

Pre-merge author checklist

• I’ve followed MetaMask Contributor Docs and MetaMask Extension Coding Standards.
• I’ve completed the PR template to the best of my ability.
• I’ve included tests if applicable.
• I’ve documented my code using JSDoc format if applicable.
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I’ve manually tested the PR (e.g., pulled and built the branch, ran the app, tested the code being changed).
• I confirm that this PR addresses all acceptance criteria and includes the necessary testing evidence, such as recordings or screenshots.

[github-actions]

CLA Signature Action: All authors have signed the CLA. You may need to manually re-run the blocking PR check if it doesn't pass in a few minutes.

[georgewrmarshall]

Moved to README.mdx

[metamaskbot]

Builds ready [fd8acc7]

• builds: chrome, firefox
• builds (beta): chrome
• builds (flask): chrome, firefox
• builds (MMI): chrome, firefox
• builds (test): chrome, firefox
• builds (test-flask): chrome, firefox
• build viz: Build System
• mv3: Background Module Init Stats
• mv3: UI Init Stats
• mv3: Module Load Stats
• mv3: Bundle Size Stats
• mv2: E2e Actions Stats
• code coverage: Report
• storybook: Storybook
• typescript migration: Dashboard
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6
  • common: 0, 1, 10, 11, 2, 3, 4, 5, 6, 7, 8, 9
  • content-script: 0
  • offscreen: 0
  • ui: 0, 1, 10, 11, 12, 2, 3, 4, 5, 6, 7, 8, 9

Page Load Metrics (2122 ± 101 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	328	2641	2011	427	205
		domContentLoaded	1808	2632	2088	204	98
		load	1822	2644	2122	210	101
		domInteractive	32	107	55	19	9
		backgroundConnect	10	94	29	23	11
		firstReactRender	54	277	139	54	26
		getState	5	61	21	21	10
		initialActions	0	1	0	0	0
		loadScripts	1345	2123	1580	194	93
		setupStore	12	98	38	29	14
		uiStartup	2051	2905	2428	261	125

Bundle size diffs

• background: 0 Bytes (0.00%)
• ui: 0 Bytes (0.00%)
• common: 0 Bytes (0.00%)

[vinnyhoward]

LGTM🫡 Ty for cleaning my mess. Next component I make in the library won't be like this