Implement Jenkins LTS Upgrade Guide

[krisstern]

What feature do you want to see added?

Implement Jenkins LTS Upgrade Guide https://www.jenkins.io/doc/upgrade-guide/ in Antora. Reference source code can be found at https://github.com/jenkins-infra/jenkins.io.

Upstream changes

No response

[biru-codeastromer]

Hi Kris, I believe since we discussed this to be in antora we might need to change the title and description to say antora when we start antora again in july . What u think?

[krisstern]

Yes, I agree! Updated just now

[biru-codeastromer]

Thanks

[Eyepatch5263]

Hey @krisstern I wanted to contribute to this, can you assign this issue to me?

[Eyepatch5263]

@krisstern I have raised a PR for this same issue, can you kindly review it. Thanks.