Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

The Atom feed fails validation:

• https://docusaurus.io/blog/atom.xml
• https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fdocusaurus.io%2Fblog%2Fatom.xml

The error:

id must be a full and valid URL

The same issue exists for RSS.

I've been able to patch it on my own blog using this: https://github.com/johnnyreilly/blog.johnnyreilly.com/pull/357/files#diff-a6704bb15d419cd24c7a855d8188b83b36aa4a6011c40b63c8dc9f5bb60fc2ed

It would be better to fix at source.

Reproducible demo

No response

Steps to reproduce

See description

Expected behavior

Passes validation

Actual behavior

Fails validation

Your environment

• Public source code:
• Public site URL:
• Docusaurus version used:
• Environment name and version (e.g. Chrome 89, Node.js 16.4):
• Operating system and version (e.g. Ubuntu 20.04.2 LTS):

Self-service

• I'd be willing to fix this bug myself.