[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]

Made a small change and pushed to re-run the failing tests in case they were intermittently flaky, but they are still failing.

[steveburnett]

Closing in favor of #23787.