Improve docs regarding "host" profile tool-requires

> # Simplified Summary for Future Readers
>
> For anyone who finds this issue (#18148) later while trying to enforce Ninja usage for both project and dependency builds, hopefully this simplified summary helps navigate the complexities we encountered.
>
> ## Our Goal
>
> We aimed for fully reproducible cross-platform builds using **Ninja**, managed by Conan via `tool_requires`, as the exclusive CMake generator for *all* components (our main project and any dependencies built from source via `conan install --build=missing`). We wanted to avoid relying on system-installed Ninja.
>
> ## The Problem We Faced
>
> Our initial attempts failed. Dependencies either didn't use Ninja (defaulting to Makefiles) or failed because CMake couldn't find the Ninja executable during the dependency build step, even when Ninja was listed as a `tool_requires` in our main recipe or configured via profile `[conf]` settings in various ways.
>
> ## The Key Insight
>
> The crucial point, clarified by @memsharded, relates to Conan's **Host vs. Build contexts**.
> * Dependencies (like libraries your project uses, e.g., `fmt`, `log4cplus`) are part of the **Host Context**.
> * To successfully build a package *for the Host Context*, the necessary build tools (`[tool_requires]`) AND the configuration instructing the build system what to do (e.g., `[conf] generator=Ninja`) must BOTH be provided by the **Host Profile**.
> * Simply putting the `tool_requires` in the main project's recipe or in the *Build Profile* was insufficient to make Ninja available *and* active during the dependency build phase.
>
> ## Working Solution Steps (Conan 2.15.1)
>
> The configuration that ultimately worked reliably was:
> 1. **Remove Ninja** `tool_requires` from the main project's `conanfile.py`. Let the profile handle it entirely.
> 2. **Create a project-local Host Profile** (e.g., `.conan/profiles/host_ninja`). This keeps the configuration with the project.
> 3. **Inside this Host Profile file** (`host_ninja`):
> * Include the user's base settings: `include(default)`
> * Add the **configuration** to *use* Ninja:
> ```
> [conf]
> tools.cmake.cmaketoolchain:generator=Ninja
> ```
> * Add the **tool** itself to be *available*:
> ```
> [tool_requires]
> ninja/1.12.1
> ```
> 4. **Run** `conan install` specifying this profile for the host context (e.g., `conan install ... -pr:h /path/to/.conan/profiles/host_ninja`). Let the build profile default or be specified separately if needed.
> 5. **Run CMake configure** for the main project directly, passing `-G Ninja` and the path to the `conan_toolchain.cmake` generated by `conan install`. No special environment sourcing was needed in the calling script for this final step because the toolchain file contained the necessary hints.
>
> ## Disclaimer
>
> This summary reflects our understanding based on resolving *this specific issue* in our environment with Conan 2.15.1. The concepts might evolve, or there could be nuances not covered here. **Please always refer to the official Conan documentation as the definitive and most up-to-date source.** We hope this helps others facing similar challenges.
>
> ## Acknowledgements
>
> We'd like to express our sincere gratitude to @memsharded and the Conan team for their outstanding support in resolving this issue. Their expertise and willingness to guide us through the complexities of Conan's profile system was invaluable.
>
> We're sharing this guide in good faith to pay it forward to the community. If you're struggling with similar Ninja integration challenges, we hope our experience saves you time and frustration. Open source thrives on knowledge sharing, and we're grateful to be part of a community where solutions can benefit everyone.

_Originally posted by @evilrix in [#18148](https://github.com/conan-io/conan/issues/18148#issuecomment-2818456814)_

Start by finding the documentation page covering profiles and tool_requires, then compare it with the issue's Conan 2.15.1 workflow. Review the mentioned conanfile.py, .conan/profiles/host_ninja, and conan_toolchain.cmake references. Done means the docs clearly explain host-context tool availability, the Ninja generator configuration, and the relevant conan install invocation.

Written by the indexing model from the issue text.