docs: improve README and XML doc completeness

[stevehansen]

README gaps

• Exception hierarchy: No documentation of AsicTimestampException → TimestampAuthorityException, InvalidAsicContainerException, AsicVerificationException. Consumers don't know which exceptions to catch.
• TSA fallback list: TimestampAuthorityUrls option is never mentioned. No appsettings.json example showing "TimestampAuthorityUrls": ["...", "..."].
• Stream-based CreateAsync: Not shown in the Quick Start (only byte[] and file-path overloads).
• UseNonce and RequestSignerCertificates options: Both exist on AsicTimestampOptions but are never mentioned in the README.
• AsicVerifyResult.DataBytes: Not mentioned — useful for consumers who want data without calling Extract() separately.
• Verification behavior: No explanation that a single failing step causes IsValid = false, or that Error is a join of all failed step details.

XML doc comment gaps

Several IAsicService methods have only a summary — missing <param> and <returns>:

• CreateAsync(Stream, ...)
• CreateFromFileAsync
• VerifyFile
• CreateExtendedFromFilesAsync
• RenewAsync(Stream, ...)
• RenewFileAsync

Other gaps:

• VerificationStep — Name, Passed, Detail have no XML docs
• TimestampChainEntry — no class-level summary
• AsicTimestampException subclasses — no <exception> tags on throwing public methods
• AsicVerifyResult.DataBytes — null for ASiC-E not documented
• ServiceCollectionExtensions parameterless overload — doesn't mention defaults (DigiCert, SHA-256)