Refresh the documentation landing page and the roadmap

[vahid-ahmadi]

Three stale documents a JOSS reviewer reads early, found in a pre-submission review.

docs/home.md — the landing page said the API reference does not exist

In full, before this PR:

This includes example notebooks, and in the future will also include function documentation.

#324 added docs/api.md. So the first page of the documentation told a reader the function documentation was still to come. A reviewer scoring the "functional documentation" checkbox reads this page first.

It now says what the package does, gives the pip install microdf-python line, shows a three-line example (which I ran — median 30000, gini 0.2829), and links to both the examples and the API reference.

docs/examples.md — a dangling reference

Opened with "See these rendered Jupyter notebooks for examples of microdf usage" followed by no links to any notebook. Now links the one notebook that exists. The weight-preservation section below it is untouched.

ROADMAP.md — stale in both directions

• It listed "Standard error calculations for surveys with replicate weight files" as future work. That shipped in Estimate variance from replicate weights #320 and is the headline feature of the JOSS paper, so a reviewer comparing the paper against the repo would find the repo disowning it.
• It claimed the package provides "distributional tables, graphs, and special functions for working with PSL Tax-Calculator". None of that is present: grep -ril taxcalc microdf/ returns nothing, and there is no plotting code in the package.

Replaced with what the package actually provides and three things it genuinely does not do yet — design-based variance from stratum and cluster identifiers, dataset presets, and wider pandas shape-change coverage.

No package code changes.