Rename Toolkit {open,high,low,close}_at accessors as {open,high,low,close}_time

[rtwalker]

Description

We're renaming these functions in toolkit

Links

See timescale/timescaledb-toolkit#527

Writing help

For information about style and word usage, see the style guide

Review checklists

Reviewers: use this section to ensure you have checked everything before approving this PR:

Subject matter expert (SME) review checklist

• Is the content technically accurate?
• Is the content complete?
• Is the content presented in a logical order?
• Does the content use appropriate names for features and products?
• Does the content provide relevant links to further information?

Documentation team review checklist

• Is the content free from typos?
• Does the content use plain English?
• Does the content contain clear sections for concepts, tasks, and references?
• Have any images been uploaded to the correct location, and are resolvable?
• If the page index was updated, are redirects required
  and have they been implemented?
• Have you checked the built version of this content?

[github-actions]

Please allow 10 minutes from last push for the staging site to build. For internal reviewers, check web-documentation repo actions for staging build status. Link to build for this PR: http://docs-dev.timescale.com/docs-rw-ohlc-rename