create a new folder for storing designs and design decissions

[jepegit]

We need an additional folder in .issueflows for storing designs and design decisions, as well as what the project decides are good practices.

Tasks

1. Implement the creation of the folder (during issue-flow init) and make sure it is not overwritten during issue-flow update
2. Update the relevant slash commands to encourage updating / adding documents to the folder, as well as using the documentation when planning and implementing solutions to issues.

[jepegit]

Also, remember to update the documentation