Improve the lightbox on try-matrix

I noticed the "close" x in the lightbox is always top right of the screen, i.e. sometimes very far from the actual image or even over it's border. Could maybe improve the placement some time.

Also the lightbox seems to make it into browsing history, so browser navigation after checking out all the lightboxes is pretty annoying.

_Originally posted by @HarHarLinks in https://github.com/matrix-org/matrix.org/pull/3239#pullrequestreview-4083602770_

Start at the try-matrix lightbox implementation and inspect how the close control is positioned and how opening a lightbox affects browser history. Done means the close control stays near the image across layouts and browsing through lightboxes does not make back navigation unnecessarily revisit each lightbox.

Written by the indexing model from the issue text.