[docs] Replace presto-router/README.txt with README.md

[steveburnett]

Description

• Replace presto-router/README.txt with README.md.
• Minor text revisions for readability.

The file was already formatted as a Markdown file, so conversion was simple.

Motivation and Context

Change suggested by @ethanyzhang.

Impact

None.

Test Plan

Created a README.md in a sandbox GitHub repository with the content of presto-router/README.txt to check the formatting as it is displayed in GitHub. Deleted the test file afterward.

Contributor checklist

• Please make sure your submission complies with our development, formatting, commit message, and attribution guidelines.
• PR description addresses the issue accurately and concisely. If the change is non-trivial, a GitHub Issue is referenced.
• Documented new properties (with its default value), SQL syntax, functions, or other functionality.
• If release notes are required, they follow the release notes guidelines.
• Adequate tests were added if applicable.
• CI passed.

Release Notes

== NO RELEASE NOTE ==

[steveburnett]

@ethanyzhang and @tdcmeehan, I opened this duplicate PR because the tests were failing on the original #23738. Please review when you have time.

[tdcmeehan]

Should we delete the old router textfile?

[steveburnett]

Should we delete the old router textfile?

Done, thanks! But now, tests are failing.

[tdcmeehan]

@steveburnett it looks like a relevant failure:

Error:  Failed to execute goal org.apache.maven.plugins:maven-install-plugin:2.5.2:install (default-install) on project presto-router: Failed to install artifact com.facebook.presto:presto-router:txt:readme:0.290-SNAPSHOT: /home/runner/work/presto/presto/presto-router/README.txt (No such file or directory) -> [Help 1]