docs: internal links target missing or incorrect anchors

Description

Three links in the English documentation point to missing or incorrect section anchors:

• models.mdx links custom providers to /docs/providers#custom, but the generated heading ID is custom-provider.
• network.mdx links port and hostname flags to /docs/cli#run, which opens the GitHub agent run subsection; those flags are under the TUI section (#tui).
• permissions.mdx links opencode run to /docs/cli#run, which also opens the GitHub agent subsection; the generated ID for the top-level opencode run command is run-1.

Plugins

None

OpenCode version

n/a (documentation)

Steps to reproduce

1. Build the docs site with bun run --cwd packages/web build.
2. Open the Models, Network, or Permissions page.
3. Follow the links described above.
4. Observe that the first target does not exist and the two #run links open the wrong command section.

Screenshot and/or share link

n/a

Operating System

n/a

Terminal

n/a