docs: list all IMAP extension features

[lKolabrodl]

Problem

The crate-level imap-types feature table stopped at ext_metadata, while Cargo.toml also exposes ext_namespace, ext_status_size, ext_utf8, and ext_within. Readers therefore could not discover all supported extension gates from the generated crate documentation.

Fix

• Add the four missing ext_* features to the table.
• Link each feature to its defining RFC (2342, 8438, 6855, and 5032).
• Keep the table order aligned with imap-types/Cargo.toml.

Closes #727.

Verification

• cargo +nightly fmt --all --check
• cargo metadata --format-version=1 --no-deps
• Compared every ext_* key in imap-types/Cargo.toml with the crate-level table: all 9 are documented, with no missing entries.
• Attempted cargo doc --no-deps --all-features -p imap-types; dependency compilation could not start on this Windows host because the system MSVC linker (link.exe) is not installed. The manifest and nightly formatting checks complete successfully.

This contribution was prepared with AI assistance and manually reviewed.

[coveralls]

Coverage Report for CI Build 33283015463

Coverage remained the same at 91.489%

Details

• Coverage remained the same as the base build.
• Patch coverage: No coverable lines changed in this PR.
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	11749
Covered Lines:	10749
Line Coverage:	91.49%
Coverage Strength:	914.28 hits per line

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💛 - Coveralls