[INTERNAL] Add Output Styles details (#905)

Adds a section which describes the Build Output Styles feature and usage
variants.

Depends on:
- SAP/ui5-project#624
- SAP/ui5-cli#665

---------

Co-authored-by: Günter Klatt <[email protected]>

docs/pages/Project.md

@@ -34,4 +34,34 @@ The source directory must contain a directory structure representing the namespa
 The `module` type is meant for usage with non-UI5 resources like third-party libraries. Their path mapping can be configured freely. During a build, their resources are copied without modifications.
 
 
+## Build Output Style
+
+The _Output Style_ offers you control over your project's build output folder. Namespaces like `sap.m` or `sap.ui.core` can be streamlined, producing a more concise and flat output. For example, a resource like `/resources/sap/m/RangeSlider.js` will be transformed into `./RangeSlider.js`. And vice versa, applications that are built by default with `Flat` output, can leverage any namespaces they might have. 
+
+In the table below you can find the available combinations of project type & output style.
+
+| Project Type / Requested Output Style | Resulting Style |
+|---|---|
+| **application** | |
+| `Default` | Root project is written `Flat`-style. ^1^ |
+| `Flat` | Same as `Default`. |
+| `Namespace` | Root project is written `Namespace`-style (resources are prefixed with the project's namespace). ^1^ |
+| **library** | |
+| `Default` | Root project is written `Namespace`-style. ^1^ |
+| `Flat` | Root project is written `Flat`-style (without its namespace, logging warnings for resources outside of it). ^1^ |
+| `Namespace` | Same as `Default`. |
+| **theme-library** | |
+| `Default` | Root project is written in the style of the sources (multiple namespaces). ^1^ |
+| `Flat` | **Unsupported** ^2^ |
+| `Namespace` | **Unsupported** ^2^ |
+| **module** | |
+| `Default` | Root project is written with the [configured paths](https://sap.github.io/ui5-tooling/stable/pages/Configuration/#available-path-mappings). ^1^ |
+| `Flat` | **Unsupported** ^3^ |
+| `Namespace` | **Unsupported** ^3^ |
+
+^1^ The Output Style is only applied to the root project's output folder structure. Any dependencies included in the build would retain their `Default` output style. 
+^2^ Theme libraries in most cases have more than one namespace. 
+^3^ Modules have explicit path mappings configured and no namespace concept. 
+
+
 [**API Reference**](https://sap.github.io/ui5-tooling/v3/api/@ui5_project_build_ProjectBuilder.html){: .md-button .sap-icon-initiative }