docs: refresh legacy plugin references

[Varun5711]

Closes #6880

Summary

• add a top-level note clarifying that legacy plugins are superseded by Docker Engine's managed plugin system
• mark archived third-party plugin repositories inline on the legacy plugins page
• remove the broken infinit.sh volume plugin link

Problem

The legacy plugins page currently sends users to archived or broken third-party links without any warning, which makes the page look more current and supported than it is.

Testing

• git diff --check

[thaJeztah]

Thank you for contributing! It appears your commit message is missing a DCO sign-off,
causing the DCO check to fail.

We require all commit messages to have a Signed-off-by line with your name
and e-mail (see "Sign your work"
in the CONTRIBUTING.md in this repository), which looks something like:

Signed-off-by: YourFirsName YourLastName <yourname@example.org>

There is no need to open a new pull request, but to fix this (and make CI pass),
you need to amend the commit(s) in this pull request, and "force push" the amended
commit.

Unfortunately, it's not possible to do so through GitHub's web UI, so this needs
to be done through the git commandline.

You can find some instructions in the output of the DCO check (which can be found
in the "checks" tab on this pull request), as well as in the Moby contributing guide.

Steps to do so "roughly" come down to:

1. Set your name and e-mail in git's configuration:

   git config --global user.name "YourFirstName YourLastName"
   git config --global user.email "yourname@example.org"

   (Make sure to use your real name (not your GitHub username/handle) and e-mail)

2. Clone your fork locally

3. Check out the branch associated with this pull request

4. Sign-off and amend the existing commit(s)

   git commit --amend --no-edit --signoff

   If your pull request contains multiple commits, either squash the commits (if
   needed) or sign-off each individual commit.

5. Force push your branch to GitHub (using the --force or --force-with-lease flags) to update the pull request.

Sorry for the hassle; let me know if you need help or more detailed instructions!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[Varun5711]

Thanks for the detailed instructions, @thaJeztah! Sorry about forgetting the DCO sign-off — I've amended the commit and force-pushed the fix. The Signed-off-by line should now be in place and the check should pass shortly.

[thaJeztah]

Thanks! I see you used your GitHub handle (Varun5711); could you use your real name (I think it's "Varun Hotani") for the sign-off? Otherwise need to adjust our .mailmap file 😅

[Varun5711]

Ha, my bad! Fixed it — amended the commit with my real name (Varun Hotani) and force-pushed again. Should be good now!

[thaJeztah]

thanks! LGTM

I think this is the right balance; we listed these plugins for people to discover, and even if they're archived, some may still be good to use, or to use as an example). We may consider removing them at some point, but not urgent.