Archive v18 docs and mark it as EOL

[rohit-nayak-ps]

As part of v21 release, we now archive v18. As Vitess only supports 3 last versions, those are now v21, v20, v19.

We archive the v18 docs as per instructions in https://github.com/vitessio/vitess/blob/main/doc/internal/release/eol-process.md

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