PR: Set up design documentation and add dialogs section

[isabela-pf]

Slowly but surely, I am aiming to document design patterns across Spyder for the reference of the whole community. This is the very beginning of the process.

All of these documents are open to feedback and change. This is just the first public attempt for the project.

Changes

• Adds a design-docs directory to store this and future design-focused documentation
• Adds a readme in the design-docs directory that states a basic purpose and index for further documentation.
• Adds dialogs.md, a mostly complete documentation of the design decisions we made when working on dialogs between Spyder 4 and 5. This will also serve as the bar for similar documentation of UI elements.
• Adds an images directory for storing the images needed in the documentation. At the moment, this only holds images for the dialogs.

Questions

• Do we want to add a license to this repo as it starts to actually hold content and not just issues?

Let me know if you have any questions or other feedback! Thanks for reviewing this PR!

[CAM-Gerlach]

@ccordoba12 Should we go ahead and merge this as a starting point? Or is it no longer up to date/relevant?