Write docs for `torrust_tracker` crate

[josecelano]

Documentation for the crate: https://docs.rs/torrust-tracker/3.0.0-alpha.1/torrust_tracker/

I've started writing docs for the torrust-tracker crate.

Subtasks

• crate docs: for lib.rs mod #253
• crate docs: for tracker and bootstrap mods #256
• crate docs: for sharedmod #260
• crate docs: for servers::apismod (Tracker API) #263
• crate docs: for servers::http mod (HTTP tracker) #266
• crate docs: for servers::udp mod (UDP tracker) #271
• crate docs: for configuration package #275
• crate docs: for located-error package #277
• crate docs: for primitives package #279
• crate docs: for test-helpers package #280
• Update README #292
• Add README for packages #295