Improve documentation

[aminya]

• add a website for the documentation docs: generate documentation for project_options via Sphinx #198
• add instructions for making an executable feat: add package_project function #58
• add instructions for making a header-only library feat: add package_project function #58
• add instructions for making a normal library feat: add package_project function #58
• add instructions for using doxygen-docs target
• add instructions for using sanitizers (especially on windows)
• adding more examples for using the options:
  • using PCH headers
  • using coverage
  • overriding compiler warnings,
  • adding more warnings
  • removing certain warnings for certain targets
• documenting the individual functions (e.g. enable_doxygen) in case more control is needed
• add documentation for cross-compilation

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[abeimler]

Would be nice if we can use the github Wiki for some things.
(Readme should really be an overview and link to the Wiki, IMHO)

For example what each Option do and how to uses them...
What warnings gets enabled, link to best practices.
Use of static code analyzers, sanitizers, how to enable them and why it matters (link to best practices)

Section for some util functions like run_vcpkg()

Some more Examples, Docker and maybe some Pitfalls...

[aminya]

We can also have a dedicated website with documentation:

https://stackoverflow.com/a/32901179

https://gitlab.com/Pro1/doxygen-cmake-sphinx