Summary stats

[hynky1999]

This PR adds blocks for computing summary statistics

• Added base block for creating the summary statistics
• Added merging block for merging summary statistics computed in distributed manner.
• Blocks for: line, document, word, token, contamination level stats.
• Tests checking that both base block + merger works also tests for individual summary stats blocks.
• Example how to use
• Readme update

How stats works

• Each stat block has to implement extract_stats, which defines what statistics to track
• Statistics are then grouped based on:

• summary -> All documents together
• fqdn -> documents grouped by fqdn
• suffix -> documents grouped by suffix
• histogram -> grouping is done based on values of statistics

Misc

• fixes bug in from_dict at src/datatrove/utils/stats.py