What to #include, namespaces not obvious from the documentation

### Describe the issue

For example, on [this page](https://google.github.io/googletest/reference/matchers.html) I see uses of `HasSubstring` but nothing on that page tells me what namespace it comes from.

On [this page](https://google.github.io/googletest/advanced.html) I see

```c++
using ::testing::HasSubstr;
```

in examples, but `#include ` is insufficient to find that, and nothing on the page tells me what the right `#include` is.

### Steps to reproduce the problem

Read the docs

### What version of GoogleTest are you using?

ff233bdd4cac0a0bf6e5cd45bda3406814cb2796

### What operating system and version are you using?

macOS 14.6.1

### What compiler and version are you using?

gcc-14

### What build system are you using?

CMake 29.3

### Additional context

_No response_

Start with the matcher reference and advanced documentation pages linked in the issue, then inspect how their examples name HasSubstring and HasSubstr. Update the documentation so each matcher’s namespace and required #include are clear, and verify that the examples’ include guidance is sufficient.

Written by the indexing model from the issue text.