Improve documentation generally

[gunvirranu]

The documentation in general could be improved.

• The installation instructions are very general at the moment and assumes the reader is sufficiently familiar with build systems (CMake).
  • Make use of FetchContent_Declare more robust
  • Look into other common build systems. Would be nice to provide simple methods for those as well
• Improve structure of README and Docs page
  • Make linking them better and completely reduce duplication. Docs for code-specific stuff, README for general info.
  • Explain some technical backstory
  • Ensure the problem statement is clear, and what it provides specifically (inputs: {TLE, time}, outputs: {state vector}, embedded)