feat: translate doc to en, add index files

docs/en/index.md

@@ -0,0 +1,12 @@
+# FZLang documentation
+
+Choose a section to explore the FZLang documentation.
+
+- [The book](./book/index.md) — a guided introduction to the language.
+- [Reference](./reference/index.md) — types, operators, grammar, and runtime diagnostics.
+- [Internals](./internals/index.md) — virtual machine implementation details.
+- [Standard library](./stdlib/index.md) — built-in functions.
+- [Language philosophy](./philosopy.md)
+
+[Документация на русском языке](../ru/index.md)
+