[site] index.md: fix "new issue" link
[site] index.md: fix "new issue" link

artspb/intellij-sdk-docs,breandan/intellij-sdk-docs,chashnikov/intellij-sdk-docs,JetBrains/intellij-sdk-docs,develar/intellij-sdk-docs,chashnikov/intellij-sdk-docs,chashnikov/intellij-sdk-docs,breandan/intellij-sdk-docs,develar/intellij-sdk-docs,artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,JetBrains/intellij-sdk-docs,breandan/intellij-sdk-docs,artspb/intellij-sdk-docs,develar/intellij-sdk-docs,chashnikov/intellij-sdk-docs,artspb/intellij-sdk-docs