Add Age writing and Pages guide

Writing-Ages.md

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+# Writing Ages
+
+Rivencraft's **Symbol Pages** describe an Age. Put a Link Panel first, then add Pages to a Notebook or the Book Binder in the order you want them read. A Link-Panel-only book needs no Symbols and chooses seeded random traits.
+
+## Page families
+
+Pages can select terrain, biomes and their distribution, weather, seasons, lighting, sky effects, celestial bodies and their modifiers, structures, decorations, and block or fluid materials. The creative inventory separates these families into Page tabs; optional JEI lists generated Pages too. Blank Pages are internal bases, not craftable player outputs. The Link Panel comes from the Ink Mixer, not the Writing Desk.
+
+Supported terrain includes **Standard, Amplified, Island, Cave, Flat, and Void**. Material Pages can be placed before a compatible feature, such as a solid Block Page before Rocks or a Fluid Page before a lake feature. If a material-bearing feature has no material Page, its material is chosen from a compatible pool using the Age seed. A material Page left without a feature is paired with a seeded compatible feature.
+
+Biome Pages go before their biome controller. Modded biomes from eligible TerraBlender regions can become Pages when those optional mods are installed. Some biome/terrain and feature/material combinations are unsupported; the Binder reports a fatal incompatibility rather than silently discarding them.
+
+## Order and instability
+
+The Binder preserves written order. It can repair known misplaced or repeated choices when compiling the Age, but those choices add **instability**. Repeated feature and material Pages are intentional and can have separate effects. Valuable generated blocks also contribute live instability as chunks are explored. An unstable Age may develop decay, lightning, explosions, meteors, burning, or other hazards. The exact instability score is an **operator diagnostic**, not displayed in ordinary book text.
+
+For a technical explanation of order and supported features, see the [writing grammar](https://git.paddedcell.org/dnorberg/rivencraft/src/branch/main/docs/WRITING_GRAMMAR.md) and [current implementation](https://git.paddedcell.org/dnorberg/rivencraft/src/branch/main/docs/CURRENT_IMPLEMENTATION.md). Check their version notes when following older examples.
+
+## Seasonal Pages
+
+With Serene Seasons and GlitchCore installed, an Age can use **No Seasons**, **Seasons**, or one of **Eternal Winter, Summer, Fall, or Spring**. These operate per Age rather than changing the Overworld calendar. Without a seasonal Page, a newly created Age chooses a seeded season mode only if Serene Seasons is installed. The [season integration notes](https://git.paddedcell.org/dnorberg/rivencraft/src/branch/main/docs/SERENE_SEASONS_COMPATIBILITY.md) explain the limits.