docs: add fillTransaction public action docs

[a26nine]

Adds the missing standalone docs for fillTransaction in the public actions docs.

fillTransaction was added earlier in #4098, but it did not get a corresponding docs page/sidebar entry. This PR fills that gap by:

• adding the fillTransaction public action docs page
• linking it in the docs sidebar
• documenting node support requirements for eth_fillTransaction
• clarifying when to use fillTransaction vs prepareTransactionRequest

I considered keeping the explanation only inside prepareTransactionRequest, but a dedicated page seemed more consistent with the existing action-docs convention and with the public API/source exports.

Reviewer attention:

• wording around fillTransaction vs prepareTransactionRequest
• the node support compatibility note
• whether the sidebar placement/order is appropriate for the existing docs structure

Verification:

• pnpm docs:build

This is a docs-only change, so I did not add runtime tests.

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