Make the cache documentation even more awesome

+1, thanks for creating this issue!

I'm planning to work on this this week.

One small improvement already: #2527126-28: Only send X-Drupal-Cache-Tags and -Contexts headers when developer explicitly enables them.

Rewrote https://www.drupal.org/developing/api/8/cache/tags, and gave it a clean URL consistent with the others.

Also created https://www.drupal.org/developing/api/8/cache/max-age.

Cleaned up the child pages of https://www.drupal.org/developing/api/8/cache; the remaining ones are all solid (they're the ones mentioned in #3 + #4, plus https://www.drupal.org/developing/api/8/cache/contexts, which I've also updated slightly to be in sync with the two others, structure-wise).

Filed #2541432: Follow-up for #2231595: Document why ChainedFastBackend cannot use BackendChain because that is quite confusing ATM.

Created https://www.drupal.org/documentation/modules/smart_cache and updated/rewrote https://www.drupal.org/documentation/modules/internal_page_cache.

Rewrote https://www.drupal.org/developing/api/8/cache.

Next: work on documenting CacheableDependencyInterface + addCacheableDependency().

For example , I didn't know config can be added.

But https://www.drupal.org/developing/api/8/render/arrays/cacheability#concr... has been mentioning this all along. In the code example, we even specifically call addCacheableDependency($render_array, $config).

AFAICT you simply hadn't read that yet?

Now also created https://www.drupal.org/developing/api/8/cache/cacheable-dependency-inter.... That should address the remaining concerns.

Making one last pass now.

Made that last pass; AFAICT we're in a pretty good place now. If you have further concrete suggestions, let me know.