wiki: add Installation page

Installation.md

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+# Installation
+
+k-ap is published to a private Gitea registry. Add it via the registry (recommended) or via git.
+
+---
+
+## Via Gitea registry (recommended)
+
+Add to `Cargo.toml`:
+
+```toml
+[dependencies]
+k-ap = { version = "0.3.0", registry = "gitea" }
+```
+
+Add to `.cargo/config.toml` (create if it doesn't exist):
+
+```toml
+[registries.gitea]
+index = "sparse+https://git.gabrielkaszewski.dev/api/packages/GKaszewski/cargo/"
+```
+
+---
+
+## Via git
+
+If you don't have registry access:
+
+```toml
+[dependencies]
+k-ap = { git = "https://git.gabrielkaszewski.dev/GKaszewski/k-ap.git", tag = "v0.3.0" }
+```
+
+---
+
+## Requirements
+
+- Rust edition **2024**
+- `tokio` runtime with `macros`, `net`, `rt`, `rt-multi-thread`, `sync`, and `time` features
+- `axum` 0.8 — k-ap returns an `axum::Router`
+
+---
+
+## Current version
+
+**0.3.0**
+
+See the [Migration Guide](Migration-Guide) if upgrading from 0.1.x.