docs: add skill true frontmatter (#17877)

This adds a `skill: true` to the frontmatter of the bestpractices
skill/doc. This way when we sync we can concern ourselves to only the
documents that need to be skills (we will write one for sveltekit too)
without targeting specific files in the `ai-tools` repo

Author: paoloricciuti

documentation/docs/07-misc/01-best-practices.md

@@ -1,5 +1,6 @@
 ---
 title: Best practices
+skill: true
 name: svelte-core-bestpractices
 description: Guidance on writing fast, robust, modern Svelte code. Load this skill whenever in a Svelte project and asked to write/edit or analyze a Svelte component or module. Covers reactivity, event handling, styling, integration with libraries and more.
 ---