Issue #2934916 by japerry, joachim, moshe weitzman: docs: Rewrite README.md...

Jakob Prequested to merge
issue/memcache-2934916:2934916-update-readme- into 8.x-2.x

Issue #2934916 by japerry, joachim, moshe weitzman: docs: Rewrite README.md around a quick start, a settings reference and audience-ordered sections

The README had grown by accretion into 1,000 lines ordered by when each issue landed: a Drupal 7 disclaimer first, install steps numbered 1-4-8 that never showed a complete settings.php, eleven "advanced" subsections in no order, three bootstrap-container walls of PHP, a 2009-era troubleshooting entry and a syntax error in the server-list example.

Same facts, new shape:

  • Requirements, then a Quick start that is the whole setup for a single server, then a short "How it works" (bins/clusters/servers, sharding not replication, invalidation by timestamp) that the rest builds on.
  • A settings reference table listing every $settings['memcache'] key with its default and a link to the section that explains it.
  • Common setups grouped by situation (several servers, several web servers, a server going down, persistent connections, authentication, disabling per environment, proxies, private memcached per node).
  • Locks, flood control and the memcache cache-tag checksum together as "optional services", each with a sentence on what it is for.
  • One bootstrap container block with the pure-memcache and key-value variants as diffs, prefaced with why and when not to; the pre-enable container_yamls pattern and the chained-fast explanation from the earlier branch (both verified live) folded in.
  • Operating (status report, Memcache Admin, debugging), troubleshooting keyed by symptom, developer notes (transactions, invalidateAll(), preload, DDEV).
  • The PECL memcache extension demoted to an appendix that lists what it lacks; hash algorithm, binary protocol and TCP_NODELAY to a tuning appendix. Dropped: the Drupal 7 warning, "PHP 7.4", the d.o/node/1131458 and /273824 links, the "Failure to set key" entry, the old-wiki binary protocol link, and the claim that example.services.yml overrides bins (it does not).

Closes #2934916

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