docs: Improve code blocks

[edibotopic]

This PR improves the consistency and functionality of code blocks in the docs.

The main changes are:

• Addition of a CSS selector that allows copy buttons to be deactivated in code-blocks. This is especially useful for code blocks that contain output.

A code block with the regular behaviour is written:

```{code-block) text PS C:\Users\myuser> echo $env:USERPROFILE ```

Result (note the button on cursor hover):

If we do not want the copy button, we write:

```{code-block) text :class: no-copy C:\Users\myuser ```

Result (note the lack of button on cursor hover):

• Addition of regex-based solution to prevent copy/pasting of command prompts. This ensures that the prompt and any text preceding the prompt is not copied when a user clicks the copy button. This is done automatically for any code block marked with the {code-block} directive.

The example below shows two lines with complex prompts and a symbol sometimes used as a prompt ($) in the commands (top). After the copy button is clicked, only the relevant information is pasted into the text editor (bottom). This means that users can more easily and quickly copy commands from the docs:

Other minor changes:

• Explicit use of code block directive in markup.
• Replacement of blockquotes and other unconventional methods of code markup.
• Where possible, separation of input and output, using text to explain which is which.
• Prefer no syntax highlighting for terminal commands.
• Delete any image that duplicates content in code blocks.
• Some minor fixes to style and wording.

UDENG-3441

[edibotopic]

@CarlosNihelton --- I updated the PR with some of the following changes:

• Used "Ubuntu (Preview)", as discussed.
• Removed # as a prompt symbol. We do not (currently) use it in these docs and I think we should avoid using a third symbol if possible (if needed we can just specify root). In addition, a side-effect of including them now is that it creates problems in Python/PowerShell code blocks, where # followed by whitespace (comments) can be marked as a prompt by the regex.
• Add prompt symbols throughout the docs because I think if we're going to use them then we need to do it consistently.
• Added a section to contributing.md on how the code blocks are implemented and things that a contributor should be aware of.