en: update the index title for better display (#2645)

[ti-chi-bot]

This is an automated cherry-pick of #2645

What is changed, added, or deleted? (Required)

Removes the word "Documentation" from the banner on https://docs.pingcap.com/tidb-in-kubernetes/stable, for better visual experience

and for consistency with that on https://docs.pingcap.com/tidb/stable , which doesn't use the word "Documentation"

Which TiDB Operator version(s) do your changes apply to? (Required)

• master (the latest development version)
• v1.6 (TiDB Operator 1.6 versions)
• v1.5 (TiDB Operator 1.5 versions)
• v1.4 (TiDB Operator 1.4 versions)
• v1.3 (TiDB Operator 1.3 versions)
• v1.2 (TiDB Operator 1.2 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):

[lilin90]

/approve

[ti-chi-bot]

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