feat(theme): add option to use language as title for code blocks

[hichemfantar]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes Add Ability to Use Codeblock Language as Container Title #10808) and the maintainers have approved on my working plan.

Motivation

Currently, Docusaurus renders the language specified in a codeblock as part of its syntax highlighting, but it doesn't allow using the language as the container title. This proposal suggests adding an option to display the codeblock's language as the title of the container for improved clarity and user experience.

When documenting code examples, it's often helpful to explicitly label the codeblocks with their language for better readability and context. While custom titles can be added using the title attribute in fenced codeblocks, it would be beneficial to have a built-in feature that automatically uses the language as the container title.

Test Plan

Test links

Deploy preview: https://deploy-preview-10809--docusaurus-2.netlify.app/

Related issues/PRs

#10808

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	Report
/	🟠 72	🟢 98	🟢 96	🟢 100	Report
/docs/installation	🟠 53	🟢 97	🟢 100	🟢 100	Report
/docs/category/getting-started	🟠 73	🟢 100	🟢 100	🟠 86	Report
/blog	🟠 64	🟢 96	🟢 100	🟠 86	Report
/blog/preparing-your-site-for-docusaurus-v3	🔴 47	🟢 92	🟢 100	🟢 100	Report
/blog/tags/release	🟠 63	🟢 96	🟢 100	🟠 86	Report
/blog/tags	🟠 74	🟢 100	🟢 100	🟠 86	Report

[hichemfantar]

@slorber i could use your help understanding some of the failing jobs

[slorber]

CI fails because defining DEFAULT_CONFIG is not enough, you also have to declare a Joi schema and define defaults for themeConfig.codeBlock and themeConfig.codeBlock.useLanguageAsTitle otherwise users providing themeConfig: {} will not have these fields by default.