feat(examples): add source code view doc, add r/ README

[leohhhn]

Description

Related to #3084

Adds a realm that teaches the user about the source code viewer in gnoweb.

It also adds a r/ root README.

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests

[moul]

Regarding the view source demo, I suggest we add your realm now. We should soon have Gnoweb displaying the README file, if it exists, in the right block of the page. Then, we can invite people to click on "view source" from the README of hello, and remove this realm.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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