Fix docs site

[olim7t]

The docs site is broken:

% cd docs
% npm run build:production

> k8ssandra-docs-v2@0.1.0 build:production
> hugo --environment production

hugo: downloading modules …
hugo: collected modules in 19667 ms
Start building sites …
hugo v0.111.3+extended darwin/amd64 BuildDate=unknown
Error: Error building site: ".../k8ssandra-operator/docs/content/en/tasks/monitor/_index.md:1:1": EOF looking for end YAML front matter delimiter

• [x] fix the issue => done in 1749e92
• [x] add a step to the release instructions to run this locally before tagging, so that we find out issues before the automated CI workflow. => done in #920
• [ ] redeploy the docs for v0.4.0 manually

┆Issue is synchronized with this Jira Story by Unito ┆Issue Number: K8OP-233

[Miles-Garnsey]

@adejanovski does the docs site need to be built with a particular npm version?