Fix broken "edit this page" links in Flux CLI section

[snebel29]

This MR closes #2203

As described in the original issue, the fix to disable the source page related links is pretty simple by adding importedDoc: true within the header.

It took me a while to realise where this was exactly needed, I was unable to find documentation about that option and another while to test that the change was good.

Validation procedure

I did checkout https://github.com/fluxcd/website then I ran

$ make serve

Which generated the site and served it on http://localhost:1313 then navigated to http://localhost:1313/docs/cmd/flux/ and observed the "Edit this page" along with the other source page links.

After that edited content/en/docs/cmd/flux.md by adding importedDoc: true under title.

---
title: "flux"
importedDoc: true
---

then copy paste the hugo server from the previous make serve run to avoid overriding my change.

$ hugo server \
                --bind 127.0.0.1 \
                --buildDrafts \
                --buildFuture \
                --disableFastRender

And observed the links related to the source page (view page source, edit this page and create child page) disappear.

I think this change is good to go.

[stefanprodan]

LGTM

Thanks @snebel29 🏅