Styling improvements for landing page

## Describe the feature

Improvements to the [landing page](https://callstack.github.io/react-native-ios-kit/) styling

Stuff that feels improvable:

1. In the dark mode this dark seem practically black when set against the white screenshot - would make it a bit lighter

2. On the most left and right item the margin between it and side of the page seems a bit too small - even smaller than e.g. between the right menu options

3. These 0 px border radius corners seem very sharp when set aside the very rounded borders of the code block. Maybe add a tiny border radius to reduce the contrast?

4. Maybe make these headlines either larger or bolded? They don't stand out that much from the paragraph right now.

5. More bottom padding here? The tiny QR code seems to be shoved to the bottom by the vastness of the black background (# poetic)

6. Definitely more margin between the items and Callstack logo

And finally: 7. This `Get started` button seems to be placed a bit randomly, isn't?

## Motivation

It could look a bit better. Wdyt?