Chore: Add minor improvements to README.md

[mkopcins]

Description

Add minor improvements to README.md:

• Add additional badges (stars, documentation, and contributors)
• Center badges and library name at the top of the readme
• Change link from GH of Private Mind to landing page
• Add accordion on Table of Content
• Fix spelling of ExecuTorch in one place
• Add consistent title case for all sub-headings
• Move documentation section (Right now, there is also badge with documentation, so section about documentation doesn't need to be at the top of README.md anymore)

If every change is ok, analogical changes need to be done in different languages

Introduces a breaking change?

• Yes
• No

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

Screenshots

Related issues

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings

Additional notes