Add event markers and improve visualizations on GitHub community page

Follow-up from PR #52

Visualization improvements needed

1. Add event markers to charts

Mark important community events on timeline charts:

• Hackathons (with dates and locations)
• Meetups and conferences
• Major releases or milestones
• Training events

These markers should appear on:

• GitHub contributors chart
• Commits chart
• Issues chart (when added)

2. Split combined visualizations

Currently the "Commits and Issues" section only shows commits. Need to:

• Add a proper issues over time chart
• Keep commits and issues as separate area charts
• Show them side-by-side or in tabs
• Each should be an area chart showing cumulative growth

3. Implementation suggestions

• Use Evidence's ReferenceArea or ReferenceLine components for event markers
• Consider creating a shared events dataset that can be reused across pages
• Event markers should have tooltips with event details
• Different colors/styles for different event types (hackathon vs meetup)

Example events to mark

• Hackathons: Various locations and dates throughout the year
• Nextflow Summit events
• nf-core meetups
• Training workshops
• Major nf-core releases

Acceptance criteria

• Event markers appear on contributor growth chart
• Event markers appear on commits chart
• Issues chart is added and separated from commits
• Both commits and issues use area charts
• Event markers have informative tooltips
• Visual distinction between event types

No files or tests are named. Start by locating the GitHub community page and its contributor and commits chart components, then inspect how Evidence chart data is assembled; review ReferenceArea and ReferenceLine usage before deciding where a shared events dataset belongs. Done means separate cumulative commits and issues area charts, event markers with informative tooltips on the specified charts, and distinct event styling.