Documentation for storage type

[shreyas-damle]

Before removing deprecate warning:

(venv-3-11) ➜  pebblo git:(shreyas-documentation-changes) ✗ pebblo --config pebblo/app/config/config.yaml
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DeprecationWarning: 'file' Storage Type will be deprecated starting from Pebblo version 0.0.19, use 'db' instead
Pebblo server version 0.1.17 starting ...
Downloading topic, entity classifier models ...

After removing deprecate warning:

(venv-3-11) ➜  pebblo git:(shreyas-documentation-changes) ✗ pebblo --config pebblo/app/config/config.yaml
Pebblo server version 0.1.17 starting ...
Downloading topic, entity classifier models ...