feat: add required javascript for pagefind in desktop help pages

[nicodh]

Adds javascript needed for deltachat/deltachat-desktop#4691

[github-actions]

Check out the page preview at https://staging.delta.chat/1057/en/

[r10s]

as this script is used also for iOS and android, which already have a search, we should add the <script> tags only conditionally (similar to --add-top-links)

<title> and <span> anchor can be added unconditionally - but is <title> needed? it is not localised and seems not to be displayed at all on desktop

[nicodh]

as this script is used also for iOS and android, which already have a search, we should add the <script> tags only conditionally (similar to --add-top-links)

ah ok, didin't know that - of course I will add a condition for that

<title> and <span> anchor can be added unconditionally - but is <title> needed? it is not localised and seems not to be displayed at all on desktop

title can be removed since it's dynamically set (and localized) with Javascript onload

[nicodh]

I added the requested changes and refactored the script a bit

[r10s]

lgtm, did not test