Add high-level documentation

https://nostr-ruby.com

docs/getting-started/installation.md

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+# Installation
+
+Install the gem and add to the application's Gemfile by executing:
+
+```shell
+bundle add nostr
+```
+
+If bundler is not being used to manage dependencies, install the gem by executing:
+
+```shell
+gem install nostr
+```
+
+## Requiring the gem
+
+All examples in this guide assume that the gem has been required:
+
+```ruby
+require 'nostr'
+```

docs/getting-started/overview.md

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+---
+editLink: true
+---
+
+# Getting started
+
+This gem abstracts the complexity that you would face when trying to connect to relays web sockets, send and receive
+events, handle events callbacks and much more.
+
+## Visual overview
+
+Begin your journey with an overview of the essential functions. A visual representation below maps out the key
+components we'll delve into in this section.
+
+```mermaid
+classDiagram
+    class Client {
+        connect(relay)
+        publish(event)
+        subscribe(subscription_id, filter)
+        unsubscribe(subscription_id)
+    }
+    class Relay {
+        url
+        name
+    }
+    class Event {
+        pubkey
+        created_at
+        kind
+        tags
+        content
+        add_event_reference(event_id)
+        add_pubkey_reference(pubkey)
+        serialize()
+        to_h()
+        sign(private_key)
+    }
+    class Subscription {
+        id
+        filter
+    }
+    class Filter {
+        ids
+        authors
+        kinds
+        since
+        until
+        limit
+        to_h()
+    }
+    class EventKind {
+        <<Enumeration>>
+        SET_METADATA
+        TEXT_NOTE
+        RECOMMEND_SERVER
+        CONTACT_LIST
+        ENCRYPTED_DIRECT_MESSAGE
+    }
+    class KeyPair {
+        private_key
+        public_key
+    }
+    class Keygen {
+        generate_key_pair()
+        generate_private_key()
+        extract_public_key(private_key)
+    }
+    class User {
+        keypair
+        create_event(event_attributes)
+    }
+
+    Client --> Relay : connects via <br> WebSockets to
+    Client --> Event : uses WebSockets to <br> publish and receive
+    Client --> Subscription : receives Events via
+    Subscription --> Filter : uses
+    Event --> EventKind : is of kind
+    User --> KeyPair : has
+    User --> Event : creates and signs
+    User --> Keygen : uses to generate keys
+    Keygen --> KeyPair : generates
+```
+
+## Code overview
+
+Explore the provided code snippet to learn about initializing the Nostr [client](../core/client.md), generating
+a [keypair](../core/keys), [publishing](../relays/publishing-events) an event, and
+efficiently [managing event subscriptions](../subscriptions/creating-a-subscription) (including event reception,
+filtering, and WebSocket event handling).
+
+```ruby
+# Require the gem
+require 'nostr'
+
+# Instantiate a client
+client = Nostr::Client.new
+
+# a) Use an existing keypair
+keypair = Nostr::KeyPair.new(
+  private_key: 'your-private-key',
+  public_key: 'your-public-key',
+)
+
+# b) Or create a new keypair
+keygen = Nostr::Keygen.new
+keypair = keygen.generate_keypair
+
+# Create a user with the keypair
+user = Nostr::User.new(keypair: keypair)
+
+# Create a signed event
+text_note_event = user.create_event(
+  kind: Nostr::EventKind::TEXT_NOTE,
+  content: 'Your feedback is appreciated, now pay $8'
+)
+
+# Connect asynchronously to a relay
+relay = Nostr::Relay.new(url: 'wss://nostr.wine', name: 'Wine')
+client.connect(relay)
+
+# Listen asynchronously for the connect event
+client.on :connect do
+  # Send the event to the Relay
+  client.publish(text_note_event)
+
+  # Create a filter to receive the first 20 text notes
+  # and encrypted direct messages from the relay that
+  # were created in the previous hour
+  filter = Nostr::Filter.new(
+    kinds: [
+      Nostr::EventKind::TEXT_NOTE,
+      Nostr::EventKind::ENCRYPTED_DIRECT_MESSAGE
+    ],
+    since: Time.now.to_i - 3600, # 1 hour ago
+    until: Time.now.to_i,
+    limit: 20,
+  )
+
+  # Subscribe to events matching conditions of a filter
+  subscription = client.subscribe(filter)
+
+  # Unsubscribe from events matching the filter above
+  client.unsubscribe(subscription.id)
+end
+
+# Listen for incoming messages and print them
+client.on :message do |message|
+  puts message
+end
+
+# Listen for error messages
+client.on :error do |error_message|
+  # Handle the error
+end
+
+# Listen for the close event
+client.on :close do |code, reason|
+  # You may attempt to reconnect to the relay here
+end
+```
+
+Beyond what's covered here, the Nostr protocol and this gem boast a wealth of additional functionalities. For an
+in-depth exploration of these capabilities, proceed to the next page.