Missing read more sections in the docs

[czoido]

Collecting missing references from the docs to start completing missing parts:

In cross_building tutorial:

• Cross-build using a tool_requires``
• Using Conan to build for iOS

In building for multiple configurations tutorial

• VS Multi-config
• Example about how settings and options influence the package id -> @danimtb
  How settings/options influence package id #3623

In understanding the flexibility of using conanfile.py vs conanfile.txt

• Importing resource files in the generate() method -> Add a note about copy resources in the generate method #3620
• Conditional generators in generate()

Using build tools as Conan packages

• Using MinGW as tool_requires
• Using conf to set a toolchain from a tool requires -> @czoido

Create your first Conan package

• Create your first Conan package with Visual Studio. -> Feature/msbuild create #3615

Preparing the build

• Use the generate() method to import files from dependencies. -> Add a note about copy resources in the generate method #3620
• More based on the examples mentioned above …

Test_conan_packages

• Test tool_requires packages

Tool requires packages

• Toolchains (compilers) -> @czoido

Lockfiles

• Continuous Integrations links.

More things:

• A tutorial for a system package (like opengl, etc.) -> @czoido