VM-Specific Pragma Annotations documentation is not discoverable

[Hixie]

https://github.com/dart-lang/sdk/blob/main/runtime/docs/pragmas.md doesn't seem to be in Google. For example, if I search for "vm:platform-const-if" on Google, it doesn't come up.

[a-siva]

//cc @kevmoo @mit-mit wondering if you folks know how the file above is setup for search

[kevmoo]

We haven't done anything special to stop indexing AFAIK. Weird that it shows up our review group but not GitHub. Hrm...