Fix broken links and rendering issues in docs content

[externl]

Fixes broken link targets and markdown rendering bugs across 13 content pages, found during a full editorial review of the docs.

The biggest offenders: the slice-grammar page had three link definitions literally pointing at TODO (doc-comment, keywords, streamed-parameters-guide) plus a [preprocessed] reference with no definition at all, and two pages (slice/encoding/icerpc.md, slice/language-guide/primitive-types.md) were missing the blank line between the last paragraph and their link-definition block — so the definitions rendered as literal text and every reference link on those pages was broken.

The rest are smaller: a #request-field → #request-fields anchor, a case-sensitive #String → #string fragment, two [IFeatureCollection] API links that pointed at the FeatureCollection class instead of the interface, a missing [InvalidDataException] definition, three "cs:x"-style reference links missing their outer brackets, a category heading at the wrong level on the service-address page, and a few stray blank lines and a dangling list continuation.

Verified by checking every page link, anchor fragment, and reference definition in content/ against the site's actual route table and heading-ID generation, and by rendering the two previously-broken pages through the repo's @markdoc/markdoc.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes broken reference links, incorrect anchor fragments, and Markdown formatting issues across documentation pages in content/, improving link integrity and rendering consistency.

Changes:

• Replaced placeholder/missing reference-link definitions and corrected link targets/fragments.
• Fixed Markdown rendering issues caused by missing blank lines before reference-definition blocks.
• Corrected several reference-link syntaxes and a heading level to match document structure.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
content/slice/language-reference/slice-grammar.md	Replaces TODO/missing reference definitions with valid targets for previously broken links.
content/slice/language-guide/primitive-types.md	Adds a separating blank line so the reference definition block renders correctly.
content/slice/language-guide/operation.md	Adds spacing to avoid list/paragraph rendering issues.
content/slice/encoding/icerpc.md	Adds missing blank line before link definitions; fixes #String → #string fragment casing.
content/icerpc/invocation/service-address.md	Adjusts heading level for correct document structure.
content/icerpc/invocation/outgoing-request.md	Fixes IFeatureCollection API reference to point to the interface.
content/icerpc/ice-protocol/protocol-frames.md	Fixes request-fields anchor fragment to match the target section id.
content/icerpc/dispatch/incoming-request.md	Fixes IFeatureCollection API reference to point to the interface.
content/icerpc-for-ice-users/rpc-core/communicator.md	Fixes list formatting so the final bullet renders properly.
content/icerpc-for-ice-users/ice-definitions/operation.md	Fixes malformed reference-link syntax for "marshaled-result".
content/icerpc-for-ice-users/ice-definitions/fields.md	Fixes malformed reference-link syntax for "cs:identifier".
content/icerpc-for-ice-users/ice-definitions/enum-types.md	Fixes malformed reference-link syntax and adds missing [InvalidDataException] link definition.
content/getting-started/icerpc-protobuf-tutorial/server-tutorial.md	Removes an extraneous blank line inside a code block to improve rendering.

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[externl]

This is an unchanged context line — this PR only changes the heading level above it. The ori typo is fixed in #515 (which touches this same file).