Improve the navigation of the VeraCrypt documentation

Originally created by @sajolida on #15670 (Redmine)

Right now we have 4 sections:

• Unlocking a file container
  • Using Files
  • Using Disks
• Unlocking a partition
  • Using Files
  • Using Disks

During the user testing of the VeraCrypt beta all the participants (5/5) ended up following a wrong section of the doc. Even though they lacked a good understanding of the VeraCrypt concepts, the major problem was about navigating between the 4 sections, understanding which one was about what and making it clear what the section people were following was about.

I had the following ideas (to be tried out):

• Use longer section title like “Unlocking a file container using the Files browser”. See also #15669 (closed).
• Use drawings to explain and visualize “file container” and “partition”.
• Use toggles to only display the relevant sections. This might not be enough unless we can trigger closing down all the other sections.
• Use separate pages for different sections.
• Clarify that “partition” also applies to entire disks.

Parent Task: #14480 (closed)