Fix getting started example

[flxst]

The getting started example is updated as it does not reflect some recent changes in modalities. In particular, this PR aims to:

• [x] Give the readme file examples/getting_started/README.md a general update
• [x] Replace the previously employed tokenizer file (only tokenizer.json) by a tokenizer instance (e.g. the variant PreTrainedHFTokenizer)
• [x] Use dataset config files as input for the cli command modalities data pack_encoded_data
• [x] Update the training config file examples/getting_started/example_config.yaml
• [x] Get the cli command modalities generate_text [..] to work again (based on #102)

EDIT: Note that the above changes only make the getting started example work again. Suggestions for additional changes to improve user friendliness can be found in #117.

[flxst]

The getting started example has now been updated, adjusted to all the recent changes (CLI commands, config files etc.) and it's working again. However, the parameters in the training config file still need to be improved - they are currently not equivalent to those in the old training config file, and the text generated by the example model is very poor.

[flxst]

Parameters in the training and text generation config files have been improved. Text generated by the example model seems to be according to expectations.