[Request] [fennel/en]

I am not familiar with [fennel](https://fennel-lang.org/) but I would love a high-level tour of the language for anyone reading this who is familiar.

Start with the Fennel language site linked in the issue and review existing language pages in the repository to understand the expected high-level tour format. Done means adding a concise Fennel overview under the fennel/en request, covering the language sufficiently for readers familiar with it.

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