sig-release: Organize KEPs into directory structure

[justaugustus]

• Move KEPs into new directory format
• Add symlinks for previously-created KEPs
• Clean up image links
• Split KEP metadata into their own files

Summary of directory layout:

$ tree .
.
├── 1498-kubernetes-yearly-support-period
│   ├── kep.yaml
│   ├── README.md
│   ├── upgrade-cadence.png
│   └── versions-in-production.png
├── 1729-rebase-images-to-distroless
│   ├── kep.yaml
│   ├── README.md
│   └── rebase-core-master.png
├── 1731-publishing-packages
│   ├── kep.yaml
│   └── README.md
├── 1732-artifact-management
│   ├── kep.yaml
│   └── README.md
├── 1733-release-notes
│   ├── kep.yaml
│   └── README.md
├── 1734-k8s-image-promoter
│   ├── kep.yaml
│   ├── promote-process.jpg
│   └── README.md
├── 20190121-artifact-management.md -> 1732-artifact-management/README.md
├── 20190219-publishing-packages.md -> 1731-publishing-packages/README.md
├── 20190316-rebase-images-to-distroless.md -> 1729-rebase-images-to-distroless/README.md
├── 20190331-release-notes.md -> 1733-release-notes/README.md
├── 20200122-kubernetes-yearly-support-period.md -> 1498-kubernetes-yearly-support-period/README.md
├── k8s-image-promoter.md -> 1734-k8s-image-promoter/README.md
└── OWNERS

ref: #1545, #617

I've created Enhancement issues for the KEPs that had none, which I'll backfill with info this week: #1729, #1731, #1732, #1733, #1734

/assign @tpepper @mrbobbytables @jeremyrickard @johnbelamaric
cc: @kubernetes/release-team

[justaugustus]

p.s. These are strictly format changes, not content changes, except in areas where the TOCs needed to be fixed up.

[tpepper]

/lgtm
/approve
thanks for the commit split by purpose, made the review across a thousand lines suuuuuper easy to review.

[k8s-ci-robot]

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