docs: add XML documentation for PKCS#12 store API (Batch 12a-2)

[KonradSop]

Description

Adds XML documentation to the PKCS#12 keystore API (RFC 7292) under crypto/src/pkcs:

• Pkcs12Store — class summary citing RFC 7292; documents Load and Save with MAC/password
  exception contracts; documents alias lookup, certificate chain, entry management (SetCertificateEntry,
  SetKeyEntry, SetFriendlyName, DeleteEntry) and the IgnoreUselessPasswordProperty constant.
• Pkcs12Entry — class summary for the bag-attribute base type; documents the indexer,
  BagAttributeKeys, HasFriendlyName, SetFriendlyName, and TryGetAttribute.
• Pkcs12StoreBuilder — class summary; documents Build and all configuration methods
  (SetCertAlgorithm, SetKeyAlgorithm, PBES2 PRF variants, SetUseDerEncoding,
  SetReverseCertificates, SetOverwriteFriendlyName, SetEnableOracleTrustedKeyUsage).

Key Accomplishments

• High-value user API: PKCS#12 is a common entry point for key+certificate import/export; this PR
  documents the full public surface of the in-memory store.
• Accurate exception contracts: <exception> only where guards in the method body throw
  (ArgumentNullException, ArgumentException, IOException on Load).
• Consistent style: follows Batch 12a-1 / prior docs PR patterns; lines ≤120.
• No overlap with open PRs: disjoint from docs: add XML documentation for X.509 certificate and CRL generators (Batch 12a-1) #684 (X.509 generators) and Batch 7–11.

Verification

• Build Status: dotnet build crypto/src/BouncyCastle.Crypto.csproj -c Release — 0 errors, no new warnings.
• Scope: Documentation-only; no behavioural or signature changes.

Checklist before requesting a review

• I have performed a self-review of my code
• I have kept the patch limited to only change the parts related to the patch
• This change requires a documentation update
  See also Contributing Guidelines.