docs: fix typos and grammatical inconsistencies across core documentation

[irfaan101]

Why are these changes needed?

I noticed a few minor spelling typos, grammatical errors, and inconsistencies across multiple core documentation files and fixed them to improve readability and professionalism:

• README.md: Fixed a duplicate word/typo in the maintenance banner (FrameworkAF in now -> Framework is now).
• CONTRIBUTING.md: Fixed spelling typos including verion -> version, responsibilites -> responsibilities, and securty -> security.
• FAQ.md: Fixed a grammatical typo in the heading (Who should use it 0.4? -> Who should use 0.4?).
• TRANSPARENCY_FAQS.md: Fixed the non-standard word experimentational to experimental.

No logical or functional changes, just routine documentation maintenance.

Related issue number

Fixes typos left in #7521

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[irfaan101]

@microsoft-github-policy-service agree