docs: Homepage displays getting started info only for MacOS users using brew

Right now the project homepage and README mention only how to use `neil` tool to start a project. Install instructions for `neil` is given only for Mac using the brew tool.

It took me a while to figure out how to install `neil` on Windows on Linux. From the getting started and usability perspective, simple, complete getting started instructions that work everywhere would be more helpful.

Previous info on how to use `deps-new` is hidden, not sure if it's intentional or if it's not a supported way anymore.

Please update the getting started info by either providing simple `neil` (and its dependencies) install information, and/or bring back the old `deps-new` instructions if still supported.

Review the project homepage and README getting-started sections, beginning with the existing neil installation instructions and the hidden deps-new information. Confirm supported installation paths for Windows and Linux and whether deps-new remains supported; done means the getting-started documentation covers users beyond macOS and clearly explains the available options.

Written by the indexing model from the issue text.