Added legacy js client to the doc build

[delvedor]

The legacy js client has accidentally been removed from the build in #934, this pr fixes that.

Ref: https://github.com/elastic/docs/pull/934/files#diff-4a701a5adb4359c6abf9b8e1cb38819fL354
Fixes: elastic/elasticsearch-js-legacy#3

[lcawl]

The strange thing is that I don't see a 16.x branch in https://github.com/elastic/elasticsearch-js and it doesn't show up when I do a full doc build for this PR (though it doesn't generate any errors either).

[bmorelli25]

Here's 16.x: https://github.com/elastic/elasticsearch-js/tree/16.x. Didn't know we could build from tags. Also, why it was broken before but isn't now is weird. Will try running the build too.

[bmorelli25]

This PR doesn't build for me. The build doesn't like 16.x. @nik9000 any ideas why this has broken since 7.2?

INFO:build_docs:Remote branch <origin/16.x> doesn't exist in repo elasticsearch-js at lib/ES/Repo.pm line 71.

[nik9000]

It turns out the strange thing is #973.

[nik9000]

I'm going to merge this and manually force the rebuild to take and track a "real" fix in #973.

[bmorelli25]

@nik9000, how can I get this to work locally? I still get this error when running a --rebuild:

INFO:build_docs:Error executing: git archive --format=tar -o /tmp/docsbuild/DMohvocqcd/elasticsearch-js/.temp_git_archive.tar 16.x docs in GIT_DIR /docs_build/.repos/elasticsearch-js.git

[nik9000]

rm -rf .repos/elasticsearch-js.git I believe. Tags don't always pull in properly. Sorry about the wart....