add Pagefind Search for bndtools website

[chrisrueger]

see https://pagefind.app/docs/installation/
and https://pagefind.app/docs/

On the website it looks like this:

Notice the search bar at the top of each page.

The build output looks like this:

Run ./.github/scripts/docs.sh
ruby 2.7.[8](https://github.com/bndtools/bndtools.github.io/actions/runs/13346832283/job/37278307569?pr=237#step:7:9)p225 (2023-03-30 revision 1f4d455848) [x86_64-linux]
3.1.6
Bundler version 2.1.4
Configuration file: /home/runner/work/bndtools.github.io/bndtools.github.io/_config.yml
To use retry middleware with Faraday v2.0+, install `faraday-retry` gem
            Source: /home/runner/work/bndtools.github.io/bndtools.github.io
       Destination: /home/runner/work/bndtools.github.io/bndtools.github.io/_site
 Incremental build: disabled. Enable with --incremental
      Generating... 
                    done in 3.871 seconds.
 Auto-regeneration: disabled. Use --watch to enable.

Running Pagefind v1.3.0
Running in verbose mode
Running from: "/home/runner/work/bndtools.github.io/bndtools.github.io"
Source:       "_site"
Output:       "_site/pagefind"

[Walking source directory]
Found 151 files matching **/*.{html}

[Parsing files]
Found a data-pagefind-body element on the site.
↳ Ignoring pages without this tag.

[Reading languages]
Discovered 1 language: en
  * en: 150 pages

[Building search indexes]
Language en: 
  Indexed 150 pages
  Indexed 6[9](https://github.com/bndtools/bndtools.github.io/actions/runs/13346832283/job/37278307569?pr=237#step:7:10)33 words
  Indexed 0 filters
  Indexed 0 sorts

Total: 
  Indexed 1 language
  Indexed 150 pages
  Indexed 6933 words
  Indexed 0 filters
  Indexed 0 sorts

Finished in 1.006 seconds