Rehash of statistics page

[terry1purcell]

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Improved readability of the statistics page. Including clarification of the different statistics.

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Hi @terry1purcell. Thanks for your PR.

I'm waiting for a pingcap member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[dveeden]

A few places could use links to the SQL statements (mostly places related to loading and exporting statistics)

The "copyable" lines can all be removed iirc.

There are a few 'sqlgram' uses. Would be best to replace them with the new EBNF things. #4389

[dveeden]

Adding @mjonss as reviewer for the parts that are related to partitioning.

[mjonss]

LGTM

[qiancai]

Rest LGTM

[lilin90]

/lgtm

[lilin90]

/test pull-verify

[lilin90]

/retest

[lilin90]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: lilin90

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[lilin90]

/ok-to-test