ELI5: Improve clarity of Email Routing docs (index, limits, postmaster) (#29352)

- postmaster.mdx: Expand ARC explanation with forwarding context, add
  DMARC functional definition, rewrite sender rewriting section to
  explain envelope vs header From, define in-session for SMTP errors,
  simplify RBL intro, fix grammar (don't → do not, characters → character)
- limits.mdx: Clarify allocation error as CPU/memory exceeded, add
  rule definition before limits table
- index.mdx: Add inline definition for authoritative nameserver, make
  Email Workers and Custom addresses descriptions concrete with examples