Write a technical documentation

[ckorzen]

Write a documentation containing all useful information about pdftotext++ and link to it in the README.
In particular, the documentation should contain the following information:

• the overall system architecture;
• the CI/CD pipeline executed on Github after pushing to master, and where and how it is defined;
• what services exist (apt-repo, github-release), and what they are doing
• explain the difference between graphics and figure
• explain what a segment is
• ... to be continued ...