add redirect for old docs link to new content

## Redirect old docs page to new version

```
[ ] Bug report
[x] Content update
[ ] Process update (build, deployment, ... )
```

Redirect this old dead page from the legacy docs site to the new version. 

Old: https://docs.cypress.io/guides/core-concepts/launching-browsers.html#
New: https://docs.cypress.io/guides/guides/launching-browsers.html#Browsers
