minor #16103 Add missing information about tsconfig.json (iquito)

This PR was submitted for the 5.4 branch but it was merged into the 4.4 branch instead.

Discussion
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Add missing information about tsconfig.json

When introducing TypeScript in a project I noticed the current documentation is still missing the mandatory step of including a tsconfig.json file in the project - without it the error "The 'files' list in config file 'tsconfig.json' is empty" occurs.

Commits
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9e47c94 Add missing information about tsconfig.json

Author: javiereguiluz

frontend/encore/typescript.rst

@@ -20,6 +20,11 @@ Want to use `TypeScript`_? No problem! First, enable it:
     +     //.enableForkedTypeScriptTypesChecking()
       ;
 
+Then add a `tsconfig.json` file with the contents `{}` to the project folder,
+or in the folder where your TypeScript files are located, like the `/assets`
+folder. In `tsconfig.json` you can define more options on how to process
+TypeScript, but defining no additional options will already work.
+
 Then restart Encore. When you do, it will give you a command you can run to
 install any missing dependencies. After running that command and restarting
 Encore, you're done!