feat: document_preamble lint rule

[adthrasher]

This pull request adds a new rule to wdl.

• Rule Name: document_preamble
• Rule Kind: Lint warning
• Rule Code: v1::W009
• Packages: wdl-grammar

Implements document_preamble as defined in #11.

Before submitting this PR, please make sure:

• [x] You have added a few sentences describing the PR here.
• [x] You have added yourself or the appropriate individual as the assignee.
• [x] You have added at least one relevant code reviewer to the PR.
• [x] Your code builds clean without any errors or warnings.
• [x] You have added an entry to the relevant CHANGELOG.md (see ["keep a changelog"] for more information).
• [x] Your commit messages follow the [conventional commit] style.
• [x] Your changes are squashed into a single commit (unless there is a really good, articulated reason as to why there shouldn be more than one).

Rule specific checks:

• [x] You have added the rule as an entry within the the package-specific rule tables (wdl-ast/src/v1.rs for AST-based rules and wdl-grammar/src/v1.rs for parse tree-based rules).
• [x] You have added the rule as an entry within the the global rule table at RULES.md.
• [x] You have added the rule to the appropriate fn rules().
  • Validation rules added to wdl-ast should be added to fn rules() within wdl-ast/src/v1/validation.rs.
  • Lint rules added to wdl-ast should be added to fn rules() within wdl-ast/src/v1/lint.rs.
  • Validation rules added to wdl-grammar should be added to fn rules() within wdl-grammar/src/v1/validation.rs.
  • Lint rules added to wdl-grammar should be added to fn rules() within wdl-grammar/src/v1/lint.rs.
• [x] You have added a test that covers every possible setting for the rule within the file where the rule is implemented.
• [x] You have run wdl-gauntlet --save-config to ensure that all of the rules added/removed are now reflected in the baseline configuration file (Gauntlet.toml).