Add user guide page 19: reconstruction from KEGG

[edkerk]

Adds user guide page 19, Reconstruction from KEGG — the route to a draft with
no template model, where orthology groups carry the reactions.

This page is not executed by CI

It carries skip-file, as the ftINIT page does. A KEGG reconstruction fetches
47 MB of artefacts and then takes about six minutes to assemble the draft —
and the download is not the slow part, so caching does not help. That is too much
for a per-commit check. Every number on the page was measured on a real run, and
the page says in its second paragraph that it is not executed, rather than
letting a reader assume the guide's usual guarantee holds.

The MATLAB tab documents a failure

getKEGGModelForOrganism cannot currently complete, so the tab shows the error
instead of a model:

Downloading the HMM library file... COMPLETE
Error: keggModel.mat not found at <ravenRoot>/reconstruction/kegg/keggModel.mat.
Generate it with the raven-toolbox Python package or download it via downloadRavenBinaries.

Neither remedy the message names exists: downloadRavenBinaries fetches only
{'blast+','diamond','hmmer'} by its own help, and raven-toolbox has no
reference to keggModel anywhere in src/ or docs/. The file is not in the
repository and updateDocumentation deletes it if present. Filed as
RAVEN#704, together with
two things met on the way: the 129 MB HMM library downloads even on this route,
which performs no homology search at all; and findRAVENroot prefers a stored
RAVEN.ravenPath preference over the copy actually executing, so on a machine
with two checkouts the error names a directory the user is not working in.

What the Python route gives

1357 rxns, 1502 mets, 838 genes
compartments:  1 — everything in 's'
exchange rxns: 0
objective:     none

One compartment, because KEGG describes reactions rather than cell biology. 19.3
is about that gap and about the keepIncomplete / keepUndefinedStoich /
keepGeneral flags — turning the first two off drops the draft to 1348 reactions
and 827 genes, a smaller change than most people expect.

Checks

• page correctly skipped by the runner in both languages
• check_names.py clean (eleven argument names added to the allowlist)
• site builds

The guide is now 19 pages. 185 examples still execute on every commit; pages 10
and 19 are the two that cannot.