chore: update READMEs to have clearer prerequisites

[haochizzle]

Description

• update main README for language and clarity
• update all example README's to have clearer prerequisites (based on feedback from eric)

Related Issue Or Context

eric and i ran through the examples and he was ready to give up by step 1. the fact that the dev needs to obtain base currency for gas and test tokens (from our faucet) should be a clear basic requirement. further, additional prerequisites such as 1) having your exported private key already on hand and 2) suggesting someone should run the EVM-to-Substrate example first to load some PHA tokens into their Substrate address before they run the Substrate-to-EVM example.

slash, there's still no "faucet" for PHA tokens on Substrate side.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Checklist:

• I have commented my code, particularly in hard-to-understand areas.
• I have ensured that all acceptance criteria (or expected behavior) from issue are met
• I have updated the documentation locally and in chainbridge-docs.
• I have added tests to cover my changes.
• I have ensured that all the checks are passing and green, I've signed the CLA bot