Add comprehensive docs for managing records

[patcon]

• record types
• diff between CNAME and ALIAS
• adding a redirect
• adding a subdomain
• adding a new domain
• explain email forwarding

[patcon]

#todo research/explain how ppl can include ppl in email fwds even if they don't want their email public (envvars?)

^^^caveat: still shows in DNS TXT record, just not in GitHub config