docs(plans): record step 1's MR for the hosted fill write deadline

Records step 1 of the npm hosted fill body write deadline plan, which opened as !2213 (merged).

Step 2 has no MR yet and its cell stays empty, which the plan reads as "not yet recorded" rather than "not started".

Filled from a dedicated docs(plans) branch rather than from the step MR itself: a step MR does not edit the plan file, because parallel step MRs editing adjacent rows of one table collide, and this plan keeps its two rows on consecutive lines.