Rewrite README - with new styling and shortened descriptions.

[abhigyantrips]

Summary

This PR closes #185, with the aim of making changes to the README inherited from discord.py.

The changes made to the README have mostly been related to the styling, and shortening of descriptions:

• Delete of README.ja.rst (as discussed among contributors).
• Change styling (to flat-square) and position of badges (now centered).
• Convert the links at the bottom into a centered footer.
• Change "Key Features" listed for the library (from the previously inherited pointers of discord.py).
• Add website hyperlink/redirect to the banner of the README.
• Shorten descriptions, move "Prefix Commands" example below "Application Commands".

Checklist

• If code changes were made then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running black .
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Bluenix2]

Hello, I was the one who originally inspired the original issue and I think this is nice but it still doesn't clarify what makes disnake stand out.

What does disnake improve over discord.py? Why should someone pick disnake over another fork (or even discord.py itself)?

PS: You're mixing reStructuredText and Markdown headers

[Bluenix2]

Aaaaaahh too late sorry

[EQUENOS]

You have a point, this readme doesn't "sell" disnake very well