[INTERNAL] Add FileSystem documentation page

docs/pages/FileSystem.md

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+# UI5 FS
+
+The [UI5 FS](https://github.com/SAP/ui5-fs) provides a UI5-specific file system abstraction
+
+[**API Reference**](https://sap.github.io/ui5-tooling/v3/api/){: .md-button .sap-icon-initiative }
+
+### Resources
+During the build phase, a modified resource is kept in memory for further processing in other build steps.
+
+This ensures performance, as physical read and write access for a high number of resources are kept to a minimum.
+
+The virtual file system offers an abstraction layer from the physical file system. Amongst others, it can combine a bunch of scattered file locations into a well defined, virtualized structure.
+
+### Adapters
+Adapters abstract access to different resource locations.
+
+The [memory adapter](lib/resources/adapters/Memory.js) represents a virtual file system, which maintains respective resources inside a data structure, whereas the [file system adapter](lib/resources/adapters/FileSystem.js) has direct access to the physical file system.
+
+### Resource Readers
+Maps virtual to physical paths.
+
+### Collections
+Multiple resource readers can be bundled to a collection. There are multiple types of collections which differ in their capability of having read or write access and in the order of how they obtain resources.
+
+#### Collection
+The collection has only read access.
+
+The collection takes a list of readers. Readers are accessed in parallel: the reader which returns the resource first is used.
+
+#### Prioritized Collection
+The prioritized collection has only read access.
+
+The collection takes a list of readers.
+The readers are accessed prioritized in the same order as they are passed to the collection.
+
+#### Duplex Collection
+The duplex collection has read and write access.
+
+The collection takes a single reader or collection of readers and a writer instance for writing results.

mkdocs.yml

@@ -22,6 +22,7 @@ nav:
       - Server: pages/Server.md
       - Builder: pages/Builder.md
       - Project: pages/Project.md
+      - File System: pages/FileSystem.md
     - Upgrade Guides:
       - Migrate to v3: updates/migrate-v3.md
       - Migrate to v2: updates/migrate-v2.md