Rename `Organisation` to `EventSeries`

[marcoroth]

Summary

This pull request renames the Organisation model to EventSeries to better reflect its purpose: grouping related conference editions together (e.g., RailsConf 2011-2025).

We opted for the simpler approach of renaming the existing model rather than introducing a three-level hierarchy (Organization → Conference Series → Event), as the current structure already serves the primary use case of grouping conference editions.

Current structure:

data/
├── latvian-ruby-community/      # ← event_series.slug
│   └── latviarb-meetup/         # ← event.slug (but named like a series?)
│
└── rails-foundation/                  # ← event_series.slug
    └── rails-at-scale-summit-2025/    # ← event.slug

Closes #169

Future Plans

In a future refactoring, the current Sponsor model will be renamed to Organisation to better represent actual organizations such as companies, communities, and foundations.

This model will include an enum to distinguish between different organization kinds (e.g., company, community), allowing us to properly model entities like Ruby Central, Rails Foundation, African Ruby Community, and Latvian Ruby Community.

Organizations will be linked to events through the existing involvements.yml files rather than through the data folder structure, separating the concept of "who organizes/sponsors an event" from "which event series an event belongs to."

Future structure:

data/
├── latviarb-meetup/             # ← event_series.slug (the meetup series)
│   ├── meetup-1/                # ← event.slug
│   │   ├── videos.yml           # ← holds the talks for just this meetup event 
│   │   └── involvements.yml     # ← References Organisation (Latvian Ruby Community)
│   │
│   ├── meetup-2/
│   └── meetup-3/
│
├── railsconf/                   # ← event_series.slug (the conference series)
│   ├── railsconf-2011/          # ← event.slug
│   │   ├── involvements.yml     # ← References Organisation (Ruby Central)
│   │   └── sponsors.yml         # ← References Organisation that sponsored the conference
│   │
│   ├── railsconf-2024/
│   └── ...
│
└── rails-world/                 # ← event_series.slug
    ├── rails-world-2023/        # ← event.slug
    │   └── involvements.yml     # ← References Organisation (Rails Foundation)
    │
    └── rails-world-2024/