Add GSoC 2026 contributor guidance and begin project ideas

[edwardchalstrey1]

• Adds a temporary page for GSoC accessible from the homepage/navbar which can be removed later, but is easy to find
• Includes contributor guidance - please review
• Adds project idea headers - please fill these out, change or remove
• I have filled out the draw_tree layout project idea myself - please review

The text is just a markdown document, so the diff should be easy enough to comment on/edit, but reminder on how to build the site here if helpful.

GSoC page

Homepage screenshot

[rahulsavani]

@edwardchalstrey1:
Thanks Ed. We've completed the ideas list and will now submit the GSoC application. We can update the contr. guidance and ideas still, so any comments welcome.