clarify build step after yarn install

[PlayPGC]

This PR improves the Getting Started documentation by clarifying the build step that should be run after yarn install.

Added a note about running yarn build after dependencies are installed

Helps new contributors avoid confusion when following setup instructions

Keeps documentation up-to-date with the actual project workflow

[PlayPGC]

@anton-trunov

Hi 👋, I made this small documentation update to clarify the build step after running yarn install.
Hope this helps new contributors avoid confusion.
Thanks for reviewing! 🙏

[novusnota]

Hey, I don't think that this separate file is needed at all — we have a general audience-friendly section on top: https://github.com/tact-lang/tact#compiler.

And when it comes for contributors building Tact compiler, it's better to add couple of lines here: https://github.com/tact-lang/tact/blob/main/dev-docs/CONTRIBUTING.md#building-tact

Like, to add the following right after the heading and before the "The most up-to-date recipe to build..." line:

```shell
# Installs necessary dependencies
yarn install

# Runs the full build, preparing tests for future runs with yarn test
yarn build

# Simplified, sped-up build of the compiler only, without tests or miscellaneous extras
yarn build:fast
```