Disable markdownlint for docs

[koppor]

We use Gitbook for our documentation. Each time when editing, new markdownlint errors appear.

Examples:

docs/advanced-reading/fetchers.md:34:3 MD038/no-space-in-code Spaces inside code span elements [Context: "`SOURCE `"]
docs/advanced-reading/fetchers.md:38:3 MD038/no-space-in-code Spaces inside code span elements [Context: "`UNKNOWN `"]
docs/advanced-reading/fetchers.md:64 MD040/fenced-code-language Fenced code blocks should have a language specified [Context: "```"]

Although I like having the same style on each text file (.java, .md, ...), I also agree that I cannot fight against Gitbook to have "proper" markdown generated.

Examples for "their" markdown:

* Open the Gradle Tool Window with the small button that can usually be found on the right side of IDEA or navigate to **View | Tool Windows | Gradle**.
*   In the Gradle Tool Window, press the "Reload All Gradle Projects" button to ensure that all settings are up-to-date
...
4. Create a run/debug configuration for the main class `org.jabref.gui.JabRefLauncher` and/or for `org.jabref.gui.JabRefMain` (both can be used equivalently)
   *   Remark: The run/debug configuration needs to be added by right clicking the class (e.g. JabRefLauncher or JabRefMain) otherwise it will not work.

I think, my energy is spend better at other places, so I disable checking that for our docs.

Nevertheless, I keep it intact for the files we create for ourselves, so that

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• 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.