Add new Persistent Object Caching section to Optimization documentation

[mxbclang]

As discussed in #234, @OllieJones has proposed a new Persistent Object Caching section for the Optimization guide. I've added the proposed content to this Google doc.

@tillkruss @felixarntz Can you please take a look and leave comments and any suggested edits? Thanks!

[tillkruss]

Will do!

[mxbclang]

Thanks for reviewing, @tillkruss! I've accepted your (excellent) suggestions. @OllieJones Heads up that Till has suggested removing the recommendations for W3 Total Cache and LiteSpeed Cache as they're quite large plugins that do a lot more than just object caching. I agree with that recommendation, but let us know what you think.

@felixarntz No huge rush, but can you please take a look at this as well when you get a moment? Thanks!

[mxbclang]

Reassigned to myself to update.

[mxbclang]

@OllieJones @tillkruss These changes have been made to https://wordpress.org/support/article/optimization/ as part of WordPress/Documentation-Issue-Tracker#415. Thank you both for your help in putting this together and reviewing! Closing this issue.

[tillkruss]

Great work!

Reading this and seeing all those links, maybe we can simplify the 2nd bullet item.

The first one Memcached Object Cache is great, but the Redis Object Cache could be made simpler with a single link:

[Redis Object Cache](https://wordpress.org/plugins/redis-cache/) – Provides a
persistent Redis object cache backend for WordPress that works with various
Redis clients. A Redis Server is required.

[mxbclang]

@tillkruss Agreed and updated, thank you!