Add ADR-0042 and fix ADR-0036

[InAnYan]

• Added a link to JavaFX Markdown UI component, that is the only one existing (for current moment).
• Wrote ADR-0042.
• Fixed some wording and style.

I don't know, but can I replace whole sections in one ADR by linking to whole sections in other ADR?

Mandatory checks

• I own the copyright of the code submitted and I licence it under the MIT license
  - [ ] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
  - [ ] Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
  - [ ] Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[github-actions]

You modified Markdown (*.md) files and did not meet JabRef's rules for consistently formatted Markdown files.
To ensure consistent styling, we have markdown-lint in place.
Markdown lint's rules help to keep our Markdown files consistent within this repository and consistent with the Markdown files outside here.

You can check the detailed error output by navigating to your pull request, selecting the tab "Checks", section "Tests" (on the left), subsection "Markdown".

[koppor]

I don't know, but can I replace whole sections in one ADR by linking to whole sections in other ADR?

That would be more than "Markdown". We need to check AsciiDoc or other templating engines. Filed at adr/madr#179.