Add guide on Hacktoberfest related issue labels

Recommended labels and their colors to denote issues that are welcoming contributors.
Feel fee skipping any.

- up-for-grabs (`#66aa66`) (this label is tracked well via popular Hacktoberfest and similar aggregators)
- Hacktoberfest (`#ed9509`)
- Difficulty: beginner(`#55ee99`) | Difficulty: junior (`#55aa66`) | Difficulty: advanced (`#5566aa`) | Difficulty: complex (`#aa3333`)
- Documentation (`#0075ca`) | Content (`#5599ee`) | Translation (`#7775ca`)
- Assigned (`#ca75ca`)
- invalid (`#eeee66`) (required to mark invalid PRs as per Hacktoberfest regulations)

Use the issue body as the source for the proposed labels, colors, and Hacktoberfest context. Decide where the guide belongs in the repository, include the relevant recommendations, and consider the instruction that labels may be skipped. It is done when the repository contains a clear guide covering the selected labels and their colors.

Written by the indexing model from the issue text.