Coveo: Implement param for left-aligned input on search page

[TasoOneAsia]

Proposed changes

This PR allows for theme consumers to add a search input directly to the search page; allowing for usage in cases where there isn't one present in the nav/header.

An example of the param usage was added to the exampleSite and resembles the following surface:

search.md

params:
  searchOnPage: true

When not supplying the page param, previous page behavior and layout is preserved.

searchOnPage Param Toggled (Theme Consumer)

searchOnPage Param Toggled Off (Example Site)

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

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