update the randomness docs

[eshaben]

Description

This PR updates the randomness docs by moving the lottery example to the tutorials section and creating a simple example on the precompile page for generating a random number

Checklist

• If this requires translations for the moonbeam-docs-cn repo, I have created a ticket for the translations in Jira
• If images have been added, I have run the compress-images.py script to compress the images.
• If this page requires a disclaimer, I have added one

After Translation Requirements

• Will need to create PR in moonbeam-docs repo to remove images
• Will need to create PR in moonbeam-docs repo to remove snippets

Items to be Updated

• Remove the RandomnessLotteryDemo.sol contract from the code snippets
• Replace the randomness images with the ones in the new directory

[albertov19]

Left some feedback. Looks good! I liked the challenges