Fix inconsistency of frontmatter "keywords" values

[gdevillele]

Problem description

Context: some pages have a keywords entry in their frontmatter header.

• The type of those entries is not consistent across the repository
• It does not make sense to have an array-typed value as there is no value in having several keyword meta tags in the pages served
• The frontmatter parser that is used currently, in some cases, fails to build the correct html meta tag.

https://docs.docker.com/docker-trusted-registry/support/ contains:

<meta name="keywords" content="Docker, supporthelp">

https://docs.docker.com/engine/swarm/how-swarm-mode-works/pki/ contains:

<meta name="keywords" content="dockercontainerclusterswarm modenodetlspki">

Problem location

In markdown files, across the whole repository.

Project version(s) affected

current stable

Suggestions for a fix

Fixing the content would be a good start.

I am going to submit PRs soon.