Docs - README.md - tweaks (thumbnaill, separators, ToC)
Docs - README.md - tweaks (thumbnaill, separators, ToC)

**this PR does basically aim at:**
- *adding thumbnail logo to README.md*
- *adding separators to README.md*
- *adding ToC to README.md*

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**main motivaton behind such:**
- *improving overall document accessebility*

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