This script provides a function for building a pkgdown site with
multiple versions.

Versions are defined by:
  - ref - a git ref to build. In the gist, prefix tags with `refs/tags/`
    and branches with `refs/remotes/origin/`
  - href - a URL path where the version will be available at
  - label - a label to display in a version switcher. Using true will
    take version from DESCRIPTION and append "(dev)" suffix

The script works as follows:

1. Copy repository into a temporary directory
2. Read `pkgdown/navbar_template.html` and populate switcher options
   with versions
3. For every version*:
    a. Create a git worktree based on version's ref
    b. Populate the navbar template with current version's label
    c. Write the navbar template to `pkgdown/templates/navbar.html`
       relative to the git worktree
    d. Copy `pkgdown/extra.css` from the repository we are building out
       of (this ensures a consistent and up-to-date styling)
    e. Use pkgdown to build site
    f. Remove the worktree

* Version with "/" href will be built first. This avoids an error from
  pkgdown regarding building the site to a non-empty directory (that's
  not managed by pkgdown).

The resulting build has the site with "/" href in the destination
directory. Respective versions are stored in subdirectories named after
labels defining versions.

Known issues
  - Every version has its own `sitemap.xml`
  - Every version has separate assets (Bootstrap, favicons, ...)
  - All versions are indexed by search engines
  - None of the versions has canonical URLs
  - "Source" link in vignettes and function reference is hardcoded to
    HEAD (instead of a respective version)
  - Switching a version redirects to index (instead of staying on the
    same subpage in target version if it exists)

This change pins bslib version to 0.5.1 to keep Bootstrap 5.2.2.
Switching to a newer bslib (and Bootstrap) version requires fixing CSS.