docs(office): fix size limit paths and document remaining env vars

Addresses the review on #15414.

The "Size limits" walkthrough described opening a stored document as a
browser upload and walked through all four variables in one sequence. The
Document Server downloads such a file itself, so the two request-body
limits never apply. Split into the two paths that actually exist:

- documents the server downloads (FILECONVERTER_MAX_DOWNLOAD_BYTES,
  FILECONVERTER_INPUT_LIMIT_UNCOMPRESSED)
- files posted to the server (NGINX_CLIENT_MAX_BODY_SIZE, MAX_FILE_SIZE)
  for image inserts, save-back, and conversion or command requests

USE_UNAUTHORIZED_STORAGE was documented as allowing HTTP storage. It sets
rejectUnauthorized: false on every outbound HTTPS connection, disabling
certificate, host name, and expiry validation, and has no effect on plain
HTTP. Now carries a warning naming the MITM exposure.

Adds the operator-facing variables the table skipped: SSL_CERTIFICATE_PATH,
SSL_KEY_PATH, SSL_DHPARAM_PATH, SECURE_LINK_SECRET, METRICS_HOST/PORT/PREFIX,
REDIS_SERVER_USER/DB, AMQP_VHOST/URI, NGINX_WORKER_CONNECTIONS,
ADMINPANEL_ENABLED, EXAMPLE_ENABLED, DS_LOG_LEVEL. SSL_VERIFY_CLIENT and
both HSTS variables are noted as inert unless SSL_CERTIFICATE_PATH and
SSL_KEY_PATH are set and both files exist, since documenting them without
their gate led readers to expect an effect.

Also notes that FILECONVERTER_MAX_DOWNLOAD_BYTES is silently ignored when
given a unit suffix, that FILECONVERTER_INPUT_LIMIT_UNCOMPRESSED replaces
the whole limit list, and documents /var/www/euro-office/Data, whose loss
regenerates the JWT secret and breaks the connector.

Tables are grouped by concern, since one flat table had become unreadable.

Signed-off-by: Christoph Schäfer <christoph.schaefer@nextcloud.com>
Assisted-by: Claude Opus 5 (1M context)
Signed-off-by: Christoph Schäfer <christoph.schaefer@nextcloud.com>

Author: chrip