en: Update info on EKS and dashboard access (#1212)

[ti-chi-bot]

This is an automated cherry-pick of #1212

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

• Add more details about accessing TiDB with an nlb with internal scheme
• Link to info about using TiDB Dashboard
• Update info about TiDB Dashboard with Kubernetes

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

• master (the latest development version)
• v1.1 (TiDB Operator 1.1 versions)
• v1.0 (TiDB Operator 1.0 versions)

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

• examples: Update AWS ELB config tidb-operator#3974
• TiDB Operator - Cannot connect to TiDB Service for TiDB Cluster External IP tidb-operator#3941

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[qiancai]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• qiancai

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[qiancai]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: ffa4962