tour: document modern way to start offline tour

[crisman]

In 2018 tour/README.md got a update to look like the then existing blog/README.md and the section got accidently placed under the patching header (wrong location) and lost the header it had in blog/README.md. At the time it was also still a python app.yaml command which, later updated to be a "go run .", which no longer matches with the install instructions for the tour which generate a "tour" binary file to run.

Add "Running Locally" header in (matching old blog/README.md style) after the "Download/Install" and move local run instructions to that new section while changing command to "tour" that is installed.

[gopherbot]

This PR (HEAD: 2e6b2cc978aa946bb81b37bac2287980331bbace) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/website/+/476298 to see it.

Tip: You can toggle comments from me using the comments slash command (e.g. /comments off) See the Wiki page for more info