x/pkgsite: add directories to "Jump to" list

[fzipp]

For modules with many packages/directories like the standard library It would be helpful if directories were listed in the "Jump to" dialog.

Example use case

Visit https://pkg.go.dev/std
Press "f"
Type "template", and the "Jump to" dialog would list:

• html/template
• text/template
• text/template/parse

[jamalc]

The jump to button in the left sidebar shows up even if a page only has directories while the "f" key shortcut is disabled, this was brought up in #45462. We should put the button in a disabled state while we explore the idea of adding directories to the jump to modal.

[gopherbot]

Change https://golang.org/cl/309489 mentions this issue: content/static: disable jump button for pages without docs

[jamalc]

What should be the behavior of the 'Jump to' button and modal when the modal won't contain any identifiers

Empty 'Jump to' modal

The empty state appears directory pages (e.g. https://pkg.go.dev/mvdan.cc/sh or https://pkg.go.dev/std) and packages with little or no documentation (e.g. https://pkg.go.dev/mvdan.cc/sh/_js)

Also, it might be more useful to explore the idea including headings from the README and directories. In that scenario the empty state would almost never come up.

Adding @georgehu and @fflewddur for UX input.

[fflewddur]

Rather than disable the "Jump to" control when the modal would be empty, including an explanation in that modal that this package doesn't have any [list of things that are "jump to"-able] would help people understand why "jump to" isn't working on certain pages. I suspect if we just disable the control, we'll end up with bug reports that the control is disabled when people expect it to be available.

Long-term, I like the idea of using this control for quick keyboard navigation of the entire page, but agree that we'd want to explore this more first.