[docs] add link from headless-cms guide to creating plugins guide

[gillkyle]

Description

Feedback from the docs in the headless CMS page asked about how one would add a new source plugin for a CMS if it didn't already exist. Reviewing the page, it mentions how to add new guides, but it seems appropriate to also add a note that links to the Creating a Source Plugin guide like we do in other places.

What to do

• Add a note to the Headless CMS page (maybe at the bottom?) that says something similar to the note in the Sourcing from Hosted Services page.

I'm marking this as claimed to give a chance to @mendaomn to claim it if he wants, who I told in another issue (#16945) that I'd help find another issue to work on

[AhsanAyaz]

@gillkyle
If @mendaomn won't take it. I will grab it 😂

[mendaomn]

Sure, I'd be more than happy to work on this!

[mendaomn]

Hey, I opened a PR to address this issue (#18432). Hope it helps!