docs: add documentation page about sending and testing emails

[enginydigital]

What

Adds a new docs/mailer.md page covering how email works with this template:

• How the Symfony Flex recipe wires Mailpit in compose.override.yaml when
  installing symfony/mailer, and how to access its web UI (the ports are
  exposed without a fixed host mapping, which regularly confuses users).
• The correct MAILER_DSN for development (smtp://mailer:1025), with a
  warning about the common 127.0.0.1 pitfall (Connection could not be established with host "127.0.0.1:1025": stream_socket_client(): Unable to connect t o 127.0.0.1:1025 (Connection refused) #723).
• Production setup: the Mailpit service only exists in
  compose.override.yaml, so nothing intercepts emails in production and a
  real transport must be configured via MAILER_DSN (Mail production #830, discussion Mail production #831).
• A short note about reusing Mailpit on staging environments as a safety net.

Why

Email configuration is a recurring source of confusion in the issue tracker
(#225, #723, #830, discussion #831). The Flex recipe does the heavy lifting,
but none of it is documented here, so users don't know Mailpit is already
available, how to reach it, or what changes in production.

The page follows the style of the existing docs (mysql.md, xdebug.md) and
is linked from the README docs index (added after "Support for extra services"):

4. [Sending and testing emails with Mailpit](docs/mailer.md)

[dunglas]

Maybe could we drop this snippet, so it doesn't get stale?

[enginydigital]

• YAML snippet: good catch — since the recipe adds the mailer service automatically, I dropped the snippet so it can't get stale.

[dunglas]

It's not done by the recipe?

[enginydigital]

• DSN: the recipe only sets MAILER_DSN=null://null (symfony/mailer 4.3 manifest), it doesn't point it to the mailer service, so this manual step is needed to actually deliver mail to Mailpit. I've made that explicit in the doc.