add multidb tutorial

docs/source/multidb.md

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+# Multidb module for database comparison
+
+With nkululeko since version 0.77.7 there is a new interface named multidb, which lets you compare several databases.
+
+You can state their names in the [EXP] section and they will then be processed one after each other and against each other; the results are stored in a file called heatmap.png in the experiment folder.
+
+>> YOU NEED TO OMIT THE PROJECT NAME!
+
+Here is an example of such an INI file
+
+```ini
+[EXP]
+root = ./experiments/emodbs/
+#  DON'T give it a name, 
+# this will be the combination 
+# of the two databases: 
+# traindb_vs_testdb
+epochs = 1
+databases = ['emodb', 'polish']
+[DATA]
+root_folders = ./experiments/emodbs/data_roots.ini
+target = emotion
+labels = ['neutral', 'happy', 'sad', 'angry']
+[FEATS]
+type = ['os']
+[MODEL]
+type = xgb
+```
+You can (but don't have to) state the specific dataset values in an external file like above.
+
+```ini
+[DATA]
+emodb = ./data/emodb/emodb
+emodb.split_strategy = specified
+emodb.test_tables = ['emotion.categories.test.gold_standard']
+emodb.train_tables = ['emotion.categories.train.gold_standard']
+emodb.mapping = {'anger':'angry', 'happiness':'happy', 'sadness':'sad', 'neutral':'neutral'}
+polish = ./data/polish_emo
+polish.mapping = {'anger':'angry', 'joy':'happy', 'sadness':'sad', 'neutral':'neutral'}
+polish.split_strategy = speaker_split
+polish.test_size = 30
+```
+
+Finally, you can run the experiment with the following command:
+
+```bash
+python -m nkululeko.multidb --config my_conf.ini
+```
+
+Here's a result with two databases.
+
+![heatmap](./images/heatmap-multidb.png)
+
+Source: http://blog.syntheticspeech.de/2024/01/02/nkululeko-compare-several-databases/