Add /ideas page

[kujenga]

Similar to #92, this is inspired by https://aboutideasnow.com/about via https://news.ycombinator.com/item?id=39511714

From that reference:

/ideas should be about the future: the crazy things people always wanted to make, concepts they're mulling over, or planned projects.
If you create an /ideas page on your website, people who are looking for collaborators are much more likely to find you.