[docs] Miscellaneous docs clean up

[colleenmcginnis]

Miscellaneous docs clean up including:

• Removing unused substitutions
• Moving images per Add table of contents scope management and refactor diagnostics docs-builder#774
• Clean up redirecting links
• Clean up Asciidoc-style links

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @colleenmcginnis? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[colleenmcginnis]

@kilfoyle we didn't backport to 9.0, right?

[kilfoyle]

@colleenmcginnis No, backporting hadn't occurred to me so #42897 is only in main.

I guess it needs to be backported but I'm a little nervous about that given the complexity. Shall we try?

I'm actually not sure if the plan is to serve the docs out of main or 9.0.

LGTM for the changes. Thanks!

[colleenmcginnis]

@rdner can you please take a look?

[kilfoyle]

@leehinman Would you maybe be able to approve and merge this? I think it's a bit urgent.

[leehinman]

@leehinman Would you maybe be able to approve and merge this? I think it's a bit urgent.

approved, but I don't see a merge "button".

[colleenmcginnis]

I disabled auto-merge and the button came back, but I get this when I try to click it:

[karenzone]

I tried and got the same message, @colleenmcginnis