Add collapsible per-venue acceptance statistics with source links

[Copilot]

This PR adds a structured way to present conference acceptance statistics (e.g., submissions, acceptances, acceptance rate) directly in each venue card, hidden behind an arrow toggle to keep the main deadlines view uncluttered. It also introduces a data source contract so yearly stats and references can be maintained consistently.

• UI: collapsible stats panel per conference

  • Added a native <details>/<summary> block under conference metadata in index.html.
  • Shows a per-year table with: Year, Submissions, Accepted, Rate, Source.
  • Renders a clear fallback when no stats exist for a venue.

• Data model: centralized stats dataset

  • Added _data/conference_statistics.yml as the canonical mapping:
    • key: conference name
    • value: list of yearly stat records with optional source URL

• Styling: lightweight table/readability updates

  • Added focused CSS in static/css/deadlines.css for spacing, compact table rendering, and discoverable summary interaction.

• Docs: contributor-facing format guidance

  • Extended README.md with the stats schema and an example entry for maintainers.

ICSE:
  - year: 2025
    submissions: 123
    accepted: 30
    acceptance_rate: "24.4%"
    source: https://example.com/source

[imranur-rahman]

Based on the deadlines.yml file, I want you to find the statistics about the papers of the conferences.

[imranur-rahman]

The URLs from openresearch does not really show the data, either find out correct url or find out other venues. You can do web search to figure this out. Also look at the comments in the issue to find such urls.

[imranur-rahman]

Let's focus on four main conference ASE, FSE, ICSE, and ISSTA and find out the historical data from two sources.

1. https://github.com/Xovee/cs-conf-stats and https://csconfstats.xoveexu.com/?conf=ASE
2. https://csconferences.org/ and https://csconferences.org/#FSE

We also need to show visualization similar to the csconferences website (maybe even refer to their website and get the visualization straight from there).