Use MockFileSessionStorage with Symfony framework

[BurningDog]

Show how to use MockFileSessionStorage with the Symfony framework.

[BurningDog]

It's my first documentation PR, so I hope I've done it correctly. I did find the process confusing, however,

The Edit this page link at https://symfony.com/doc/current/components/http_foundation/session_testing.html forked the repo for me, and checked out the 6.0 branch. However, the README says to use the 4.4 branch. When creating the PR I get this:

If your pull request fixes a BUG, use the oldest maintained branch that contains
the bug (see https://symfony.com/releases for the list of maintained branches).

If your pull request documents a NEW FEATURE, use the same Symfony branch where
the feature was introduced (and `6.x` for features of unreleased versions).

The proposed changes only apply to Symfony 6 - I'm not sure of the Symfony 5 syntax for introducing test overrides in yaml config files, while I know that in Symfony 4 there's are separate config files for test overrides. So I'm using the 6.0 branch.

It's also my first time using reStructuredText. There was no guidance at https://symfony.com/doc/current/contributing/documentation/format.html for what IDE to use, so I used VS Code with some recommended plugins - lextudio.restructuredtext, trond-snekvik.simple-rst, ms-python.vscode-pylance, ms-python.python.

However, there were many errors when viewing an existing rst file, e.g. Unknown directive type "configuration-block".snooty(DocUtilsParseError)

Although they did render the changes, they were full of errors e.g. Unknown interpreted text role "class".

The local build is broken.

I would appreciate some guidance on how to ensure that what I'm submitting is at least correct reStructuredText.

[Update: the lint bot - DOCtor-RST - on the PR gave output I could use to correct my changes].

[OskarStark]

Thank you @BurningDog for your contribution! We're closing this PR due to lack of recent activity and interest. If you'd like to pursue this documentation addition, please feel free to open a new PR against the 6.4 branch.