Adapt disposable documentation up to 4.3

- Cleared misunderstandings of disposable templates, this word was
  sometimes used interchangeably with disposables;
- Deduplicate content as much as possible;
  - Reference other pages or previous sections;
  - Assume the user has read the previous sections up until the
    current section they are reading;
  - All this deduplication enables easier reading, less clutter, but
    each section doesn't stand by itself, each page does. Some things
    were reinforced on multiple sections, but limited to when
    extremely necessary to learn by reinforcement;
- Structure the files to their distinct use cases:
  - how-to-use-disposables is just the basics to learn what are
    disposables and how to use them, using GUI and CLI alternatives;
  - disposable-customization has everything advanced related to
    disposables, such as advanced usage, creation of disposables
    templates, their customization;
  - disposable-implementation assumes the user understand the previous
    pages and contains a more technical description of disposables and
    preloaded disposables as well as their implementation;
- Prepared usage for a GUIVM setup, avoiding mentions to dom0;
- Usage modes has been organized in GUI first and CLI second, grouped by
  origin such as GUIVM and app qube;
- Usage page restricts itself to "default-dvm", it's name is not
  variable like the Whonix disposable template variant and it is the
  most used disposable template, thus we avoid variables replacement
  that the user should think of the value to a definitive value to use
  when learning. Customization page could not benefit much from this
  though, as it is intended to create alternative disposable templates;
- Usage page links to Tails documentation of why using it on a VM is not
  amensiac, so it can be referenced for users that ask to have Tails in
  a VM for anti-forensics purposes;
- Updated the images to R4.3, nobody deserves to see the Qubes version
  of the Xfce application menu anymore when the new app menu rocks. The
  images also had to be updated because it was fullscreen screenshot
  on a large resolution, making it very difficult to read unless
  opening the image in a new tab and zooming in;
- Implementation page shows preloaded disposables alternatives that were
  once used or considered as a comparative for future studies, to answer
  why a different option is not being used and what requirements a
  replacement must meet;
- Content from previous releases have been deleted;
- Updates to Qrexec policy v4;
- Use rST roles;
- Change non-ASCII quotes and em-dashes to ASCII;
- Standardized text writing style, many people have contributed to these
  pages over the years, there is a lot of different writings styles
  which make the text difficult to map. I rewrote a lot of the
  paragraphs to my liking and understanding of the different stages that
  a user may go through when using disposables, and even on paragraphs
  that I didn't completely write, it was modified to follow the same
  standard with the rest of the pages. The usage must be very simple
  with just the basics, we don't want to scare the user's off of using
  disposables, we want to convince them. The customization is a bit more
  advanced, it assumes a lot more knowledge from the user to make
  decisions by themselves such as customizing applications and qube
  settings, which can affect system security. The implementation details
  do not require reading the code, it just exist to explain what is
  being used and why it was chosen.

Fixes: QubesOS/qubes-issues#10282
For: QubesOS/qubes-issues#1512

Author: ben-grande