Document NBP publication schedules in client XML docs

[milten89]

Fixes BACKLOG P1 item 10.

Publication schedule (Europe/Warsaw time)

Source: NBP's "Informacja o terminach publikacji kursów walut". The site blocks automated browsers, so I read it through the search engine's copy.

Data	Published
Table A (mid, common currencies)	business days, 11:45–12:15
Table B (mid, less common currencies)	Wednesdays, 11:45–12:15; the previous business day if Wednesday is a holiday
Table C (buy/sell)	business days, 7:45–8:15
Gold	business days. No hour is documented: I couldn't confirm one from NBP's own text.

Changes

• Every method of the three client interfaces has <remarks> with the schedule of the table it reads (mid methods: A/B, buy-sell methods: C, gold), and each interface summarises it.
• Today methods explain that until that day's publication, and on days without one, the result is a NotFoundError. They point to the matching Latest method with <see cref>. Latest methods explain that they return the previous value until the next publication.
• "Can return no data" summaries now name the NotFoundError.
• Concrete client classes (NbpCurrencyExchangeRateClient, NbpExchangeRateTableClient, NbpGoldPriceClient): all 25 public methods use <inheritdoc/>, the classes inherit the interface summary, and the constructors are documented. Without this, callers using the classes directly (the ADR-0007 default) saw none of the docs; CS1591 is suppressed, which hid the gap.
• Grammar fixes ("the todays", "the gold price are").

Merge note: this PR and #22 remove adjacent lines in docs/BACKLOG.md (items 10 and 11), so whichever merges second will show a one-line conflict. Ask me to update that PR and I'll resolve it.

Verification

• dotnet build OpenUrzednik.slnx: 0 errors, no cref warnings.
• dotnet test -f net10.0: Core 90/90, Nbp 556/556, IntegrationTests 47 passed / 9 skipped (manual tests).
• dotnet format --verify-no-changes: clean.
• reviewer agent: no blocking findings. I fixed its should-fix point (<inheritdoc/> on the concrete clients) and nits (Latest wording, gold wording, NotFoundError in summaries, grammar).

🤖 Generated with Claude Code