docs: Update docs/README.md and Hugo Relearn to 5.23.0

[bernhardkaindl]

Minor updates of the Hugo documentation:

• The current Ubuntu snap package of Hugo is not supported by the docs.
  We should take a first minor step towards fixing this.

• doc/README.md is outdated and should be updated (new version).
  It says that the Ubuntu snap of Hugo works, but it does not anymore.
  Fix this by updating the outdated information.

• An initial fix is to update the Relearn theme from 5.20.x to 5.23.0:

  • It does not introduce breaking changes.
  • It introduces more straightforward page links and deprecates older syntax.
  • Fix the warnings by updating relative links accordingly.

A preview is available on my site: https://xenserver-next.github.io/xen-api/index.html

[bernhardkaindl]

@robhoes Can you review this PR and check if you like to merge it? (Pau and Christian approved it)

GitHub tells me that I no longer have write access and therefore cannot merge anymore.

[bernhardkaindl]

When updating to newer versions of the Hugo Relearn theme,
Hugo/Relearn warned about a few broken links.
The broken links are currently live on https://xapi-project.github.io/new-docs/,
and I tested the fixes on top of this PR. This comment gives the details about the new commit to
Fix currently broken links to toolstack/features: HA, snapshots:

The cause is simple:
The old docs site had the features at /features/HA/HA.html and /features/snapshots/snapshots.html.
But on the new-docs site, they are below toolstack/features

Fix them to target the respective new-docs pages toolstack/features/HA and toolstack/features/snapshots, respectively.

The fixes are kept as relative links, so all of them are work in 3 environments:

• In the fast Markdown review renderer in VS Code / Codium and its derivatives.
• The local hugo server renderer
• Deployment using GitHub Pages at different URLs.

Sample locations of the currently broken links for review:

• https://xapi-project.github.io/new-docs/design/multiple-cluster-managers/:
  • two times to xhad (broken link):
    1. On the top of the page: "Xapi currently uses a cluster manager called xhad."
    2. On the bottom of the page: "conform to the exact same interface as xhad."
       Fixes for review at: https://xenserver-next.github.io/xen-api/design/multiple-cluster-managers/ (working links)

Fix 3: Link from design/ocfs2 to (toolstack)/features/HA:
Old: https://xapi-project.github.io/new-docs/design/ocfs2:

• "which provides Host-level High Availability." (using ../../.., it traverses to the top-level above the /new-docs, where the user ends up on the old docs site at https://xapi-project.github.io/features/HA/HA.html

New: https://xenserver-next.github.io/xen-api/design/ocfs2 (fix for review):

• "which provides Host-level High Availability." (links to the correct page within the new docs site)

Fix 4: "Read more about Snapshots: the High-Level Feature." at on top of this page:
Old: https://xapi-project.github.io/new-docs/xen-api/topics/snapshots/ (link results in 404)
New: https://xenserver-next.github.io/xen-api/xen-api/topics/snapshots/ (review of the fixed link)

PS: I would have merged it already before this commit, but I lost the write permission needed for the merge.

[contificate]

Thanks for doing this, this is a good start in making the docs more readily buildable.