docs: write docs, add README

docs/api.md

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+# API Reference
+
+## `rensa.buildWith`
+
+The main entry point for creating a Rensa flake.
+
+### Arguments
+
+- `inputs`: The flake inputs.
+- `cellsFrom`: Path to the directory containing your cells.
+- `cellBlocks`: A list of blocks to load for each cell.
+- `transformInputs` (optional): A function to transform inputs before they are passed to cells.
+
+### Returns
+
+A standard Nix flake output set.
+
+## `rensa.build`
+
+The underlying builder function used by `buildWith`. It returns the raw Rensa output structure without the recursive update functor.
+
+### Arguments
+
+- `inputs`: The flake inputs.
+- `cellsFrom`: Path to the directory containing your cells.
+- `cellBlocks`: A list of blocks to load for each cell.
+- `transformInputs` (optional): A function to transform inputs.
+
+### Returns
+
+An attribute set containing:
+
+- `output`: The generated flake outputs.
+- `__ren`: Internal metadata about the cells and blocks.
+
+## `rensa.blocks`
+
+Helper functions to define blocks.
+
+### `simple`
+
+```nix
+simple "name"
+```
+
+Creates a block definition where the type matches the name.
+
+### `dynamic`
+
+```nix
+dynamic "name"
+```
+
+Creates a block definition for dynamic content. This allows integration of the `std` cli
+by passing through actions etc. from a cell.
+
+Currently not really used.
+
+## `rensa.select`
+
+Helper to select specific outputs from the generated flake.
+
+```nix
+select inputs.self [
+  ["cellName" "blockName" "outputName"]
+]
+```
+
+## `rensa.filter`
+
+Filters the generated flake outputs based on a predicate and paths.
+
+```nix
+filter predicate inputs.self [
+  ["cellName" "blockName" "outputName"]
+]
+```
+
+### Arguments
+
+- `predicate`: A function or boolean. If `true`, returns the attribute as is. If a function, it's used to filter the attributes of the target.
+- `target`: The flake outputs (usually `inputs.self`).
+- `paths`: A list of paths to select. Supports wildcards (`*`).
+
+## `rensa.get`
+
+Retrieves a single attribute from the flake outputs.
+
+```nix
+get inputs.self ["cellName" "blockName" "outputName"]
+```
+
+### Arguments
+
+- `target`: The flake outputs.
+- `path`: The path to the attribute to retrieve.