etcd-mixin: drop files from website, update links to point to new home

[chalin]

As things stand now, etcd-mixin is now found under https://github.com/etcd-io/etcd/tree/master/contrib/mixin (in particular note that it is no longer under https://github.com/etcd-io/etcd/tree/master/Documentation). Some cleanup is in order:

• All site links to any of the old locations of the etcd-mixin files should be updated to point to their new home under etcd-io/etcd/contrib/mixin
• Delete etcd-mixin sources found under the v3.4 folder
  • This will also ensure that the source files aren't published, as they are now (following Revise Versioning Structure v3.4.0➔v3.4 and current➔next #161) -- see config: ignoreFiles entries are invalid #189. E.g.,
    • https://etcd.io/docs/v3.4/etcd-mixin/readme/ exists and can be viewed
    • https://etcd.io/docs/v3.4/etcd-mixin/test.yaml can be downloaded
• config.toml: drop the v3.4.0/etcd-mixin rule from ignoreFiles.
  Some of the other ignore rules might be candidates for elimination too (edit: see config: ignoreFiles entries are invalid #189)
• Add rewrite rules just in case someone has been trying to access the etcd-mixin files from the website?

/cc @nate-double-u @spzala @ptabor
/assign @chalin
/tag cleanup

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A partial trail of the relocation of etcd-mixin files:

• Migrate documentation: Remove docs from etcd-io/etcd #97
• Migrate documentation: Remove docs from etcd-io/etcd etcd#12660 - Remove docs from etcd-io/etcd
• Restoring etcd-mixin folder etcd#12683
• Move Documentation/etcd-mixin files etcd#12685
• Migrate back mixin to contrib/ etcd#12686 - Migrate back mixin to contrib/
• Remove mixin from Doc etcd#12687
• Revert "Remove mixin from Doc" etcd#12688
• Rename mixin for kube-prometheus backward compatibility etcd#12689
• Remove mixin from doc etcd#12699 - Remove mixin from doc (final removal)