doc: Subscribing the events for the longer duration

[i3mSuperMan]

Summary

Current documentation on the events describes how to subscribe to the application, end-device & gateway events through CLI and API Calls.
Ref: https://www.thethingsindustries.com/docs/getting-started/events/

• CLI login with OAuth token will allow subscribing to the events for only one hour since the OAuth Access Tokens had an expiry of one hour.
• We can log in to CLI using a User API Key to avoid the problem with the expiry of the OAuth token and can log events without interruptions.

Changes

Added a section on how to subscribe to the events for a longer duration in the Working with events documentation.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Run Locally: Verified that the docs build using make server, posted screenshots, verified external links.
• New Features Marked: Documentation for new features is marked using the new-in-version shortcode, according to the guidelines in CONTRIBUTING.
• Style Guidelines: Documentation obeys style guidelines in CONTRIBUTING.
• Commits: Commit messages follow guidelines in CONTRIBUTING, there are no fixup commits left.