Update networks.md configuration & add initial feature matrix

[azf20]

• split mainnet and testnet into dedicated files
• separate "how to" information from configuration
• retain networks.md with pointers to the new files
• add feature support matrix

Benefits:

• separate instructions from configuration
• prepare for addition of L2 with its own config

[azf20]

This is now ready for review, given the Council approval of the feature matrix cc @fordN @Jannis

[fordN]

Looking good! Just a small typo to fix before merging. Could you also squash or cleanup commit history before rebase & merge to keep the overall history meaningful?

[azf20]

Done, thanks so much @fordN!