[docs]: add Flex configurator

[khalibloo]

Description

Added a configurator to the Flex component docs.

Before submitting the PR, please make sure you do the following

• [x] Read the Contributing guide
• [x] Prefix your PR title with [@svelteui/core], [@svelteui/actions], [@svelteui/motion], [@svelteui/core], [core], or [docs].
• [x] Include a description of the changes made in the PR description and in the commit messages.
• [x] Include screenshots/videos of the changes made.
• [x] Verify that the linter and tests are passing, with yarn lint and yarn test or just run yarn prepush.

[BeeMargarida]

I'll check this out today or tomorrow!