Remove "project model" since it's already a chapter, add more descriptions in the sdk section
Remove "project model" since it's already a chapter, add more descriptions in the sdk section

artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs,artspb/intellij-sdk-docs