docs: add custom titlebar example

* docs: add custom titlebar example

* docs: add links and other small edits

* docs: add panel window docs

* docs: remove panel example

* docs: specify expected emphasis style

* docs: responding to feedback

* docs: fix section names in links

* docs: rework baseWindow note

* docs: making window customization its own section

* responding to feedback

Co-authored-by: Anny Yang <anny@electronjs.org>