Backwards compatibility with previous AIP index pages

[rhamiltonsf]

After migrating our AIPs to the new site generator, I immediately got a an error when trying to go to the main aip index (/general). The cause for the error is that the index page general.md -- because it ends with .md is being viewed as a AIP instead of the new scope.yaml that appears to define the index pages now.

Recognizing that there needs to be a migration the aip_index files to the new scope.yaml files, and recognizing that we might be the only consumer that needs to go through this migration process right now, I feel that we should maintain support for the old aip_index format as a matter of backwards compatability.

My primary motivation for this is that in addition to our core aip_index files, we also have three code blocks with separate code owners. For me to do the migration from aip_index to scope.yaml I would have to make the changes for them, and then wait until the volunteers from the core AIP editors and the three sub group owners can approve the changes before I can merge.

I would prefer to upgrade the core AIPs, and have them be an example to the sub groups who can upgrade to the new format at their leisure, recognizing that they do not get the benefits of the upgraded format (such as extension) until they upgrade to that format.

Additionally, I think this would just be a good best practice going forward, for the core infrastructure to support one or two previous versions of the AIP structure/format. This would allow sub-orgs and AIP maintainers a bit more flexibility when doing upgrades.

[lukesneeringer]

This seems reasonable. In general, I have tried to do this (e.g. by making the old single-page AIPs work), but I thought that one was small enough not to be a huge deal. Happy to support this though.

[NAWABDERA1]

Thank you indeed