update doc to have varnish config tabs

[dbu]

now that @xabbuh added it, we should do something like https://github.com/symfony/symfony-docs/pull/4287/files and then move all varnish examples into tabs. i hope this works together with ..literalinclude

[xabbuh]

@dbu I've never used literalinclude before and don't really know what it does. But probably it's relatively easy to update the extension if that's necessary.

[dbu]

rather than copy configuration into the doc, we include the actual files
that we use for the integration tests, so we know they really work with
varnish. thats what literalinclude does, "copy" the file content in place.

[xabbuh]

Thanks for clarifying. I'll have a look at how hard it is to make it available inside configuration blocks.

[xabbuh]

It does work as it is now if you explicitly set the language used to render the contents like this:

.. literalinclude:: /foo.php
    :language: php

Is that sufficient or do you need the default language to be picked automatically if none is given?