docs: Migrate all references to our old community Slack to the new GitHub Discussions (#1776)

Author: UncannyBingo

README.md

@@ -1,4 +1,4 @@
-[Formance Stack](https://formance.com) • [![Ledger Stars](https://img.shields.io/github/stars/formancehq/ledger?label=Ledger%20stars)](https://github.com/formancehq/ledger/stargazers) [![License MIT](https://img.shields.io/badge/license-mit-purple)](https://github.com/formancehq/ledger/blob/main/LICENSE) [![YCombinator](https://img.shields.io/badge/Backed%20by-Y%20Combinator-%23f26625)](https://www.ycombinator.com/companies/formance-fka-numary) [![slack](https://img.shields.io/badge/slack-formance-brightgreen.svg?logo=slack)](https://bit.ly/formance-slack)
+[Formance Stack](https://formance.com) • [![Ledger Stars](https://img.shields.io/github/stars/formancehq/ledger?label=Ledger%20stars)](https://github.com/formancehq/ledger/stargazers) [![License MIT](https://img.shields.io/badge/license-mit-purple)](https://github.com/formancehq/ledger/blob/main/LICENSE) [![YCombinator](https://img.shields.io/badge/Backed%20by-Y%20Combinator-%23f26625)](https://www.ycombinator.com/companies/formance-fka-numary) [![GitHub Discussions](https://img.shields.io/github/discussions/formancehq/stack)](https://github.com/orgs/formancehq/discussions)
 
 Formance is a highly modular developer platform to build and operate complex money flows of any size and shapes. It comes with several components, that can be used as a whole as the Formance Stack or separately as standalone micro-services and libraries: