[Docs] Replace placeholder URLs

[kilfoyle]

Summary

In light of finding abuse of dummy links in our docs, this replaces placeholder URLs with non-hostable segments.

Generally, I tried to remove http and https protocols from dummy links. However, in some cases that change would have seriously impacted the usefulness of the docs, so I wrapped the dummy segment in angle brackets, so it's at least not a valid URL.

Rel: elastic/docs-content#1799

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[github-actions]

🔍 Preview links for changed docs

• docs/reference/elasticsearch-plugins/cloud/ec-plugins-guide.md
• docs/reference/elasticsearch-plugins/manage-plugins-using-configuration-file.md
• docs/reference/elasticsearch-plugins/plugin-management-custom-url.md
• docs/reference/elasticsearch/command-line-tools/saml-metadata.md
• docs/reference/elasticsearch/configuration-reference/machine-learning-settings.md
• docs/reference/elasticsearch/configuration-reference/monitoring-settings.md
• docs/reference/elasticsearch/configuration-reference/security-settings.md
• docs/reference/elasticsearch/rest-apis/reindex-indices.md
• docs/reference/enrich-processor/uri-parts-processor.md
• docs/reference/query-languages/kql.md