Fix Typo and Improve Documentation Consistency in README.md

[KaranDasA1]

#248 This pull request addresses minor issues in the README.md file for Stardoc:

Typo Correction: The heading "Maintaner's guide" has been corrected to "Maintainer's guide." Link Verification: Ensured that all links, including the build status badge and external references (e.g., Starlark), are accurate and functional. Heading Consistency: Updated the documentation to ensure consistent use of heading levels, particularly under the "Get Started" and other major sections. These changes improve the clarity, usability, and professional presentation of the documentation.

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[tetromino]

@KaranDasA1 - do you want to sign the Google CLA, or do you prefer to abandon this PR?