Add mod documentation in seven languages

docs/en.md

@@ -0,0 +1,74 @@
+# Menu — main menu, game choice, saves and settings
+
+Menu is the first screen you see: the title menu with **Play**, **Load**, **Settings**, **Mods & Collections** and **Exit**. It lets you pick one of the installed games, start it or continue where you stopped, load a save, and change the language, sound and display settings. Other mods can add their own buttons and settings pages to it.
+
+It belongs to the shell that is always loaded, so it is there before any game starts.
+
+![The main menu: Play, Load, Settings, Mods & Collections and Exit on the left, the Cthangover title art on the right.](assets/main-menu.jpg)
+
+## Start a game
+
+**Play** opens **Choose a game**. Every game from your collections is a card with its cover, description, readiness and number of saves.
+
+![Two game cards in Choose a game: Cthangover and My Novel, each "Ready to play", "No saves", with New game and Configure buttons.](assets/game-picker.jpg)
+
+- **New game** starts the game from its first scene. The menu switches to the game's set of mods in the same window — there is no restart.
+- **Continue** on a card resumes that game's latest save; **Saves** lists them.
+- **Install missing** appears when the game needs mods you do not have; it downloads them.
+- **Configure** opens the game's scheme in the mod manager.
+
+While you look at the menu, the games' mods are prepared in the background; the hint at the bottom says when starting will be quick.
+
+**Continue** in the main menu itself appears once you have played: it returns to the last game and save you used.
+
+## Load a save
+
+**Load** shows the saves of one game at a time; pick the game in the selector at the top and click a save to load it.
+
+![The Load window for My Novel with one save, "Day 1, 00:00", and the hint "Click a save to load it".](assets/load.jpg)
+
+If the game's mods changed since the save was made, a **The game has changed** window lists what was added (starts from scratch), removed (its data is kept and returns if turned on again) or updated, and makes a backup of the save before loading. Saves from before games existed appear under **Saves without a game**; you choose the game they belong to and the file moves there.
+
+During a game, saving and loading use the same window through the in-game menu: click a slot to save, a used slot is overwritten after confirmation, **+ New save** adds one.
+
+## Settings
+
+**Settings** has four tabs. Changes are applied and saved at once.
+
+- **General** — interface language.
+- **Sound** — on/off and volume for Sounds, Music and Ambience.
+- **Display** — window mode (windowed, maximized, borderless or exclusive fullscreen), monitor, window size, V-Sync and frame rate limit, plus **Reset display settings**.
+- **Mods** — settings pages that installed mods add, with **Reset to Defaults**.
+
+![The Display tab: window mode, monitor, window size, V-Sync and frame rate limit.](assets/settings-display.jpg)
+
+![The Mods tab: the Menu mod's own page with "Data of disabled mechanics" and a Purge button.](assets/settings-mods.jpg)
+
+Menu's own page on the **Mods** tab is **Data of disabled mechanics → Purge**: it removes from every save the data of mods that are no longer part of the games. Use it only when you are sure: a mechanic turned on again starts from scratch.
+
+## What Menu relies on
+
+Menu draws the screens and decides what each button does. The game list and its readiness come from the collections; starting, switching and saving games are done by the engine's game session and save services; the music of the title screen is the `MainMenu` playlist of Music Pack, which Menu requires. **Mods & Collections** is supplied by Mod Manager.
+
+## For mod authors: add a button or a settings page
+
+The buttons come from a registry, not from a fixed screen, so a mod can add, replace or reorder them. Mod Manager adds its button with a `main_menu` entry in its manifest:
+
+```json
+"main_menu": [
+  {
+    "id": "mods_hub",
+    "scene_path": "scenes/mods_hub_screen.tscn",
+    "button": { "icon": "mod_manager", "locale": "mod_manager/menu_title", "index": 100 }
+  }
+]
+```
+
+A mod's settings appear on the **Mods** tab when the mod registers settings definitions in the mod settings registry, as Menu does for its Purge action. Text on buttons and settings comes from the mod's own localization.
+
+## If something goes wrong
+
+1. **No games under Play.** No collection of kind *game* is installed or enabled; open **Mods & Collections**.
+2. **A game card says Missing mods or Incompatible versions.** Use **Install missing**, or **Configure** to see the problem.
+3. **Continue is missing.** It appears after you have played a game at least once.
+4. **The window does not keep its size.** Check **Display → Window mode**; window size applies only to windowed mode.