add content-type and body-validation explanation in _index.md

[gforns]

Description

request-validation limitations are not listed anymore since 3.4 version. Adding them in the main page

b492d3e#diff-f88716a1e934cd881cb07ecb1cc41ebdcd62fd950aa0e821603755e1a8caa2d4

New feature in 3.6 to perform body validation on +json content-types.

Testing instructions

Preview link:

Checklist

• [ x ] Review label added
• [ x ] Conditional version tags added, if applicable.

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[lena-larionova]

Some minor edits, and need to add charset to the dictionary. I'll handle getting that done.