Add LearnWeb3's testnet faucet to the docs

[haardikk21]

• Adds THIRDPARTY_FAUCET_LINKS. Future faucets can add themselves to this array to be included.
• Updates the MDXRenderer to include a thirdpartyFaucetLinks prop which renders a list of third party faucet links
• Updates two .mdx doc files - Building a Frontend, and Building a Smart Contract, to display third party faucet links underneath the official faucet link

[CLAassistant]

All committers have signed the CLA.

[sarahschwartz]

Thanks for this @haardikk21! For now we will just keep 3rd party faucet links on our ecosystem page, so I'm going to close this.