docs: document how to use another email catcher

[tsokolovs]

What

Adds a "Using Another Email Catcher" section to docs/extra-services.md.
It shows how to point the recipe-installed mailer service at a different SMTP
catcher, with Mailtrap Local as
the example.

Documentation only. No change to any compose file, and no new page.

Why

docs/extra-services.md records that symfony/mailer installs a Mailpit
service. The page does not say that the service is replaceable, or what a
replacement needs. The recipe writes the service into compose.override.yaml,
so the swap is a two-key edit plus the DSN.

The section also answers the port question that applies to any catcher in this
template. The recipe exposes the ports without a fixed host mapping, so
docker compose port is the way to reach the web interface.

Verified

Docker Compose v5.3.0 with mailtrap/mailtrap-local 0.4.1:

• docker compose up --wait reports the container healthy.
• A message sent from a second container to mailer:3535 arrives.
• GET /api/v1/messages on port 3550 lists that message.
• GET / on port 3550 returns the web interface.
• docker compose port mailer 3550 returns the host address.

The image binds 0.0.0.0 through its own default command, so the block needs no
command: key. The tool reads no environment variables, so the block needs no
environment: key either.

super-linter/slim@v8 ran locally with the same disabled linters as ci.yaml.
It reports "All files and directories linted successfully".

[tsokolovs]

For context: #948 adds a dedicated docs/mailer.md page for the same topic. It is approved but not merged yet, so this PR targets docs/extra-services.md, which exists today.

If #948 merges first, docs/mailer.md is the better home for this content. I have the equivalent section prepared against that page as a 25-line follow-up. I am happy to move it there and reduce this section to a one-line pointer, or to close this PR in favor of that one. Whichever you prefer.