[v8] Fix Doctests CI (#10117)

[tcsc]

The linting applied to file via the tools in next appears to vary if the files
are outside the /src directory: If the files are under /src, a rigorous lint
is applied. If the files are outside of /src, a "less-rigorous" lint is
applied, letting many legitimate issues slip through.

This patch alters the CI script to symlink the /workspace directory (the
mount-point that GCB uses to inject code into the container running a build
step) under the next image's /src/content directory, so that the correct,
rigorous lint will be applied.

It also fixes the lint errors that have crept in during the time the linter
was being incorrectly lenient.

See-Also: #9600
See-Also: #10107

Backports #10117

[zmb3]

@tcsc what PR is this the backport of?

[zmb3]

Commenting here because I got added as a reviewer and I don't see a link to the original PR.

I don't love that this creates behavior that differs locally versus when run on GCB. While I'm fine with this as a temporary fix, it would be nice to create a follow up issue to investigate a better fix.

[tcsc]

We're at the mercy of to conflicting restrictions here:

1. We can't use make -C build.assets test-docs on a buildbox image as you would locally, as this needs Docker-in-Docker for CI, which we don't want to allow, and
2. There is also no way we can customize the mount point that GCB uses (it is always /workspace), so we can't change that to move the docs source into place.

The actual test-docs task looks like this:

DOCSBOX=quay.io/gravitational/next:main

test-docs: docsbox
	docker run --platform=linux/amd64 -i $(NOROOT) -v $$(pwd)/..:/src/content $(DOCSBOX) \
		/bin/sh -c "yarn markdown-lint-external-links"

...so the build steps in the CI build file are the closest equivalent I could make that would work under GCB

[tcsc]

@tcsc what PR is this the backport of?

#10117