Add `etcdutl` documentation

[nate-double-u]

@ptabor Should we add etcdutl as well? (newly introduced in 3.5)

Originally posted by @gyuho in https://github.com/etcd-io/website/pull/288#discussion_r634007594

Potentially we could potentially add it to content/en/docs/next/dl-build.md, and/or we may want to create a new page for it.

[chalin]

Isn't it named etcdutl?

[chalin]

Ah, yes: https://github.com/etcd-io/etcd/tree/main/etcdutl

[nate-double-u]

Good catch, thanks for the correction!

[spzala]

Sharing per today's meeting discussion - https://github.com/etcd-io/etcd/tree/main/etcdutl

[nate-double-u]

/cc @Somoshree

[Somoshree]

/assign

[chalin]

For consistency, with the current site content I believe that a bullet for etcdutl should be added to Library and tools > Tools.

Oh! Now I see where the content of #478 comes from: it is from etcd/main/etcdutl/README.md.

IMHO, if we chose to add the content of https://github.com/etcd-io/etcd/tree/main/etcdutl to the website, then:

1. We should remove the content from etcd/main/etcdutl/README.md, and instead refer the reader to content on the website -- which could be under /docs/v3.5/integrations/etcdutl (See Note below.). In my opinion, this would be a good move, but it should probably be discussed at the community meeting.
2. If we do this for etcdutl then we should do it for all the tools.

Note: might some IA rework needed? Maybe /docs/v3.5/integrations/ should be split into the following (IMHO) more intuitive names especially if we bring in subpages:

• /docs/v3.5/tools
• /docs/v3.5/libraries. Come to think of it, it would actually make more sense to use /docs/v3.5/languages -- maybe it could be made similar to grpc.io/docs/languages? Then pages like Golang modules could be placed under /docs/v3.5/languages/go.

Thoughts?