Generate sitemap.xml

Description of the issue 📄

There are many 404 Not found log entries for /sitemap.xml, because search engine crawlers are trying to find one and none can be found.

Screenshots 📷

We get a lot of log lines like these

2025-09-15T19:25:35.721003+00:00 app[web.3]: [07030b65-be40-4c12-a957-4d6f2de7c23a] Started GET "/sitemap.xml" for 172.70.80.167 at 2025-09-15 20:25:35 +0100
2025-09-15T19:25:35.738385+00:00 app[web.3]: [07030b65-be40-4c12-a957-4d6f2de7c23a] Processing by ChapterController#show as XML
2025-09-15T19:25:35.738469+00:00 app[web.3]: [07030b65-be40-4c12-a957-4d6f2de7c23a]   Parameters: {"id" => "sitemap"}
2025-09-15T19:25:35.747908+00:00 app[web.3]: [07030b65-be40-4c12-a957-4d6f2de7c23a] Completed 404 Not Found in 9ms (ActiveRecord: 0.8ms | Allocations: 8081)
2025-09-15T19:25:35.750622+00:00 heroku[router]: at=info method=GET path="/sitemap.xml" host=codebar.io request_id=07030b65-be40-4c12-a957-4d6f2de7c23a fwd="216.73.216.141, 172.70.80.167" dyno=web.3 connect=0ms service=31ms status=404 bytes=0 protocol=http1.1 tls=true tls_version=unknown

These shouldn't be hitting the Rails app, nor producing any log lines.

Steps to fix 🛠

Implement sitemap.xml :-)

To do 📋

• Claim this issue (comment below, or assign yourself if you are part of the codebar org)
• Fork and clone the repository
• Update the relevant files. Follow the steps to fix section in this issue.
• Commit your changes as one commit. Use the title of this issue as your commit message
• Submit a pull request
• Mention this issue in the PR description by including it's number
• Have your pull request reviewed & merged by a codebar team member

Start with the /sitemap.xml request shown in the report, which currently reaches ChapterController#show as XML. Trace that controller and the relevant routing entry to understand how the request is handled; done means /sitemap.xml returns a sitemap rather than invoking the chapter lookup or producing a 404.