style/standardize on chain usage

[Earnwithalee7890]

What changed? Why?
Standardized the phrase "on chain" (two words) to "onchain" (one word) across documentation to align with Base's brand voice and the "Everything Onchain" messaging.
Updated instances in useWriteContract.mdx, quickstart-headless.mdx, and erc20-paymaster.mdx where "on chain" was used as an adverb or adjective.

Notes to reviewers

• Only updated instances where "on chain" functions as a compound modifier or adverb (e.g., "data onchain", "UserOperation onchain").
• Preserved correct two-word usage in prepositional phrases where grammatically appropriate (if any existed).
• Consistent with Base's established branding and style guide.

How has it been tested?

• grep search for remaining instances of "on chain" (two words) to ensure comprehensive coverage.
• Manual review of context to confirm changes align with Base's style guide.
• Verified markdown rendering in local preview using mint dev.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2