docs: say that the default questions read the visitor salt

[hughgrigg]

The prop doc on visitorSaltParameter said that a deployment whose questions count no visitors never reads it and none has to exist. That is true, and it describes a case the defaults fall outside. pageviews counts visitors, and it is one of the five questions a deployment that passes no rollups gets. The parameter is therefore a prerequisite of a default deploy, and a stack that goes up without one loses both pageviews schedules on their first call while the other four questions keep computing. The prop now says that the default questions read it, that it has to exist before the first run, and that a deployment wanting none passes rollups without a question that counts visitors. docs/summary-schedule/ says the same under the props table, next to what the permissions section already says about ssm:GetParameter reading a parameter no template creates. docs/visitors/ held the two halves of this in separate sections, and "Which questions carry a count" now carries both. The thinner copy of the doc on SummaryFunctionProps is left alone, since that construct is not exported, never sees the rollups list, and takes the parameter from whatever chose the questions. One change here is unrelated to the issue. #89 removed the viewsOnly constant from rollup-summaries.test.ts while #91 added a use of it, and main has not compiled since the two merged, with pnpm check failing in tsc before it reaches a test. That case synthesises a stack and runs no query, and it takes pageviews like the deployment case above it.

Resolves #87

• Conventional commit message, used as the title

• Conventional branch name, like feat/concise-description

• Full check with pnpm run check passed

• Rebased off latest main

• User-facing behaviour is documented in docs/

See https://www.conventionalcommits.org/en/v1.0.0/

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• docs/summary-schedule/README.md
• docs/visitors/README.md
• src/cdk/rollup-summaries.test.ts
• src/cdk/summary-configuration.ts

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