fix(content-blog): make RSS feed generation work with slugs with .html extension

[Pranav2612000]

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 #0000) and the maintainers have approved on my working plan.

Motivation

Before this commit, adding slugs ending in .html to the frontmatter blogs resulted in the build process breaking. This PR fixes this issue

Test Plan

Added Tests for the emitUtils.readOutputHTMLFile function in which the changes were to be made. Sample file also has been added in website/_dogfooding

Test links

Deploy preview: https://deploy-preview-8158--docusaurus-2.netlify.app/

Related issues/PRs

Closes #8139

[Pranav2612000]

@slorber I've created a PR. Can you take a look. I've tried to cover all these points you've mentioned

• Reproduce by adding slug: /x/y/z.html to any of our website blog post
• Add this to website/_dogfooding/_blog tests (visible here: https://docusaurus.io/tests/blog) => we'll keep this in git and ensure this issue doesn't come back
• Fix the bug in readOutputHTMLFile() + cover in unit test
• Ensure it works fine under all trailingSlash combinations (CI should tell you, we build with trailingSlash: isDeployPreview,)

[netlify]

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 63	🟢 98	🟢 100	🟢 100	🟠 80	Report
/docs/installation	🟠 79	🟢 100	🟢 100	🟢 100	🟢 90	Report

[slorber]

Thanks, LGTM 👍

Just fixed a edge case where the slug could contain .html in other places than the end