Add examples

[fulmicoton]

Right now we have one large example that does not cover all use case. It would be great to instead, have many small examples covering one single use case.

• deleting documents
• index payload and resuming indexing a log from a given checkpoint
• integer ranges
• define a custom collector
• facetting
• how to define a custom analyzer (what use case?)
• iterating through documents/position directly
• working with json

[petr-tik]

Looks like we are only missing one example showing how to resume indexing from a checkpoint.

Does tantivy support logs and checkpoints at the moment? If not, are they going to be added?