docs(adding-projects): align JSON example with documented fields; grammar and defaults clarifications​

[mshsheikh]

• Align sample context7.json with documented fields by adding branch, previousVersions.title, and a branchVersions example for parity.
• Correct wording in description and rules (“Rate limiting”, “single‑region setup”) to improve clarity and consistency.
• Standardize grammar (“works similarly to robots.txt”; use “branch‑based”; keep “cannot”) across Field Descriptions.
• Clarify excludeFiles behavior by stating that glob patterns are not supported and filenames must be exact.
• Clarify default exclusions: user excludeFiles/excludeFolders are merged with defaults; items can be explicitly re‑specified to remove from exclusion.
• Add missing sentence‑final punctuation in the Default Exclusions paragraph for consistency.

What changed

• Updated example JSON block to include branch, previousVersions.title, and branchVersions with an example title.
• Minor copyedits for accuracy and hyphenation (e.g., branch‑based, single‑region).
• Clarified behavior notes in excludeFiles and default merging semantics.

Why

• Keeps the example authoritative and self‑consistent with the fields described below, reducing contributor confusion and PR churn.
• Explicitly documents non‑glob behavior for excludeFiles and the merge semantics for defaults, preventing misconfigurations.

Notes

• Follows conventional commits style for docs changes to improve readability of history and automated changelog grouping.