Cardano Updates

Problem: several links in Markdown files are broken.

Solution:

1. Links in References.md, index.md and TechnicalReports.md
   are invalid in the sense of Markdown and this repo, but
   they work on the website, so they are explicitly ignored.

2. Glossary.md contains many links to anchors in the same file.
   These anchors work differently in Markdown and Docusarus as can
   be seen here:
   https://ouroboros-consensus.cardano.intersectmbo.org/docs/for-developers/Glossary/
   Ideally, they should be fixed to work on the resulting website,
   in which case all of them would be broken in the sense of Markdown.
   Fixing them for Docusaurus is out of scope of this commit,
   but we force xrefcheck to ignore all links in that file in
   preparation for that fix.

3. There is also an anchor link in VersioningSchemeDecision.md,
   but this file is not used in the resulting website, so we fix
   it according to Markdown rules.

4. Redirects have been replaced with direct links to make opening links
   more efficient for readers.

5. Fixed outdated links and anchors in CONTRIBUTING.md and
   ouroboros-consensus-cardano/README.md

Problem: Markdown files contain a plenty of links which
tend to get outdated occasionally. It's hard to notice when
a certain link (local or global) becomes broken.

Solution: in the past, the `xrefcheck` tool was used
to find broken links:
* https://github.com/IntersectMBO/ouroboros-consensus/pull/3
* https://github.com/IntersectMBO/ouroboros-consensus/pull/454

See https://github.com/serokell/xrefcheck
This PR automates `xrefcheck` running by adding it as a new
GitHub Action job.
It uses [xrefcheck-action](https://github.com/serokell/xrefcheck-action)
under the hood.

Resolves #4

Problem: Markdown files contain a plenty of links which
tend to get outdated occasionally. It's hard to notice
when a certain link (local or global) becomes broken.

Solution: in the past, the `xrefcheck` tool was used
to find broken links:
* https://github.com/IntersectMBO/ouroboros-consensus/pull/3
* https://github.com/IntersectMBO/ouroboros-consensus/pull/454

See https://github.com/serokell/xrefcheck
This PR automates `xrefcheck` running by adding it as a new
GitHub Action job.
It uses [xrefcheck-action](https://github.com/serokell/xrefcheck-action)
under the hood.

Resolves #4

Problem: several links in Markdown files are broken.

Solution:

1. Links in References.md, index.md and TechnicalReports.md
   are invalid in the sense of Markdown and this repo, but
   they work on the website, so they are explicitly ignored.

2. Glossary.md contains many links to anchors in the same file.
   These anchors work differently in Markdown and Docusarus as can
   be seen here:
   https://ouroboros-consensus.cardano.intersectmbo.org/docs/for-developers/Glossary/
   Ideally, they should be fixed to work on the resulting website,
   in which case all of them would be broken in the sense of Markdown.
   Fixing them for Docusaurus is out of scope of this commit,
   but we force xrefcheck to ignore all links in that file in
   preparation for that fix.

3. There is also an anchor link in VersioningSchemeDecision.md,
   but this file is not used in the resulting website, so we fix
   it according to Markdown rules.

4. Redirects have been replaced with direct links to make opening links
   more efficient for readers.

5. Fixed outdated links and anchors in CONTRIBUTING.md and
   ouroboros-consensus-cardano/README.md