Add installation and compatibility guide

Installation.md

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+# Installation
+
+Rivencraft 1.0.0.2 targets **Minecraft 1.21.1**, **NeoForge 21.1.264 or newer in the 21.1 line**, and **Java 21**.
+
+## Required mods
+
+Install Rivencraft, **Dynamic Dimensions AR Edition 0.9.1**, and **Bad Packets 0.8.2** on both client and server. A singleplayer installation needs these same required mods.
+
+## Optional integrations
+
+| Mod | Rivencraft feature |
+| --- | --- |
+| JEI | Searchable Pages and guides for writing and crafting stations. |
+| Create (includes Ponder) | Ponder destination snapshots and a portal-construction guide. |
+| Immersive Portals | Live destination views in books and crystal portals. |
+| TerraBlender | Eligible biomes from mods using its regions, such as Biomes O' Plenty. |
+| Serene Seasons with GlitchCore | Independent seasons in Ages, including eternal-season Pages. |
+| Ruins | Authorable Ruins-template Pages and generated ruins. |
+
+None of these optional mods is needed for core Age travel. **Ecliptic Seasons is not supported** for dynamic Ages in this version. The [compatibility review](https://git.paddedcell.org/dnorberg/rivencraft/src/branch/main/docs/MILESTONE_7_COMPATIBILITY_REVIEW.md) records development checks.
+
+## Before updating
+
+Stop the server and back up the **entire world folder**, not just an Age's dimension folder. Age definitions, books, and dimension data must be restored together. This is a release candidate, so old development saves are not guaranteed to work across every update.
+
+After installation, follow [Getting Started](Getting-Started). If a book opens but its Age will not load, see the [Age recovery guidance](https://git.paddedcell.org/dnorberg/rivencraft/src/branch/main/docs/AGES.md#administrator-recovery).