docs: fill out remaining external link placeholders

[tonyandrewmeyer]

All the remaining "UPDATE LINKS" notes in the docs are replaced with actual links to the external Juju/Charmcraft docs.

Also changes the "product page" to be juju.is rather than juju.is/docs/sdk as the latter is going away (juju.is was recommended by @tmihoc).

(These changes were previously in a commit in another PR but got lost when changes were forced - it seemed cleanest to break them out into a separate PR).

[james-garner-canonical]

Dropping this seems fine especially since we're going to be recommending jubilant for integration tests

[tonyandrewmeyer]

Yeah, I figure this entire page will be rewritten this cycle so there's no much point making any improvements.

[dwilding]

Thanks @tonyandrewmeyer & @tmihoc. Approving with a couple of observations, which I can revisit (please add any thoughts that you have):

• I believe that the official syntax for an intersphinx link in MyST is:

  [Juju Storage](inv:juju:std:label#manage-storage)
  

  instead of:

  {external+juju:ref}`Juju Storage <manage-storage>`
  

  I don't think there's any difference in practice, though. So I'm in two minds about whether it's worth changing all the links.

• Out of the scope of this PR, but we still have a bunch of hard-coded links to juju.is, some of which might be broken. For instance, the link to "Set up / tear down automatically" from Set up your development environment is broken. I can review and migrate these links in a separate PR.

[tmihoc]

@dwilding I was just hacking the syntax from rST (and was in fact not aware of the possibility you mention). That said, I do think the brackets syntax suggests external link whereas the one I used aligns more with the {ref} syntax for internal links. As the point of intersphinx is to make external links local, I'd favor the syntax that better suggests that. That said, I really don't have strong feelings about this.

[dwilding]

I like your reasoning, @tmihoc. FWIW, in MyST .md files I prefer [Link title](#reference) instead of the {ref} syntax, so to me the brackets do look more like internal links 😄

Anyway, again, no strong feelings. @tonyandrewmeyer I don't have any objection to merging this as-is.