feat: spis tresci w README, generowany z jego wlasnych naglowkow - #246

The owner pointed at awesome-free-apps, which has one, and asked why we did not
notice. Fair. README.md is 311 lines across nine sections and 52 of 55 unique
visitors in the last fortnight landed on it with no way to jump. GitHub has an
outline button; it is an icon most readers never press.

Generated rather than written, for a reason that cost something earlier today.
The error-page index in ERROR-MESSAGES.md was maintained by hand while the pages
beside it were generated, and eleven pages ended up in the sitemap with nothing
linking to them. A contents list has the same failure mode: it goes stale in
silence, because a missing row looks like nothing at all.

Checked rather than assumed, and this is the part worth keeping. The obvious
anchor implementation - lowercase, strip punctuation, collapse whitespace to
hyphens - produces two dead links out of nine in this file:

  "## ⭐ Support ZeroSMTP"       GitHub renders #-support-zerosmtp
  "## Security & Deliverability" GitHub renders #security--deliverability

The emoji is removed and leaves the space in front of the word, which becomes a
leading hyphen. The ampersand is removed and leaves the spaces on both sides,
which become two. So the rule is: do not strip the ends and do not collapse
runs. That was found by reading the ids off the rendered page on github.com
rather than by reasoning about it, and every one of the nine now matches an id
that actually exists there.

Not added to the eighteen pages under docs/. They are not orphans - index.md
links twenty-two of them and the layout carries navigation - so a contents list
on each would be decoration, and PRINTERS.md, the one long enough to need one,
already has it.