feat: add deduplication doc (#655)

Added small document that explains how deduplication works

content/smartmodules/deduplication.md

@@ -0,0 +1,74 @@
+---
+title: Deduplication
+---
+
+The `Deduplication` feature in Fluvio is the way to deduplicate records based on their keys.
+
+Deduplication can be used on a topic.
+It will drop the duplicate records in a window which is defined by the configured `bounds`.
+
+The current supported `bounds` are `age` and `count`, they are explained in the [bounds section](#bounds).
+
+## Behavior
+
+The deduplication function is deterministic and survives restarts. During restart the deduplication algorithm scans the data stream and rebuilds the memory object.
+
+## Example topic config
+
+Example configuration on topic:
+
+%copy%
+```yaml
+# topic.yaml
+version: 0.1.0
+meta:
+  name: topic-with-dedup
+deduplication:
+  bounds:
+    count: 5 # remember at least 5 last records
+    age: 5s # remember records for at least 5 seconds
+  filter:
+    transform:
+      uses: infinyon-labs/dedup-filter@0.0.2 
+```
+A topic can be created using this config file like so:
+
+%copy first-line%
+```bash
+$ fluvio topic create -c topic.yaml
+```
+
+After creating the topic, it can be tested like so:
+
+%copy first-line%
+```bash
+$ fluvio produce topic-with-dedup --key-separator :
+1:2
+1:2
+2:5
+```
+
+%copy first-line%
+```bash
+$ fluvio consume -B topic-with-dedup
+2
+5
+```
+
+## Bounds
+
+| Parameter       | default | type   | optional | description                                           |
+|:-------------|:--------| :---   | :---   |:------------------------------------------------------|
+| count |    -    | Integer | false | Minimum number of records the filter will remember. It doesn't guarantee to remember records that came `count` records before now. |
+| age   |    -    | Integer | true | Minimum amount of time this filter will remember a record for. It can be specified using this format: `15days 2min 2s`, or `2min 5s`, or `15ms` | |
+
+## Implementation
+
+The specific algorithm used for deduplication is defined by a smartmodule. Right now the only smartmodule for this is the `dedup-filter` smartmodule.
+
+The `dedup-filter` splits the data into smaller chunks and keeps these chunks in memory. Each chunk has an age so it knows if it can
+drop a chunk based on it's age.
+
+When count of total records in memory exceeds the configured `bounds.count`, it will check if it can delete the oldest record chunk using the configured `bounds.age` and delete it if it can. This allows for fast deletion of old data with minimum amount of bookkeeping.
+
+The chunking of data introduces a small memory usage overhead but allows the filter to have no spikes in execution time and memory usage.