Add popover for file tree browser discoverability (first time user experience)

Context

See &17781 (comment 2673324349)

Design proposal

The first time users see the file tree browser, introduce the feature by showing a dismissible popover. Pajamas feature discovery documentation

Default state & first time experience	Reference: Issues popover
File tree navigation Browse your repository files and folders with the tree view sidebar.

Interaction details

See !202647 (comment 2739756447)

• The "first time experience" popover does not appear on hover, and should not replace the tooltip that appears on hover.
• The "first time experience" popover should appear on page load with a short delay, as soon as the user sees a directory/file page with the file tree for the first time.

  • Per Pajamas:

    The popovers can be animated in with a short delay after the page has loaded to make sure that they’re noticed.

• If the user clicks on the file tree expand/collapse button, or clicks the close button on the popover, the popover disappears.