Update wiki to reflect 4.x (tracking issue)

This is the overview of the suggested/planned changes to the Wiki. Text in bold are for extra considerations/options.

• Additional-Reading.md: review links, add newer links to blogs and topics
• Alphabetical-List-of-Observable-Operators.md: remove non-standard, deleted & otherwise project-external operators, add newer operators, link to the categorized other .md files and sections in the wiki.
• Alphabetical-List-of-3rd-party-operators.md: create page and list operators that used to be in RxJava and/or interesting enough, but available in 3rd party libraries, such as rxjava2-extras.
• Async-Operators: review the operator set, point to RxJava2Extensions project, add fancy descriptions like in #6131.
• Backpressure: update wording, remove unavailable operators, fix image links, make sure examples are in Java.
• Backpressure-(2.0).md: maybe review, should be okay as it has been recently added
• Blocking-Observable-Operators.md: update to blockingX methods, add fancy descriptions like in #6131, existing operators only, fix image link
• Combining-Observables.md: expand operator list (concat/eager/withLatestFrom missing?), fancy like #6131, link to RxJava2Extensions or explain join patterns here.
• Conditional-and-Boolean-Operators.md: review operators, link/explain extensions ops, fancy like #6131
• Connectable-Observable-Operators.md: add newer operators, have examples in Java, fix image links
• Creating-Observables.md: add generate
• Error-Handling-Operators.md: review set, fancy like #6131
• Error-Handling.md: review, only existing exceptions, include the error handling explanations from the What's different page.
• Filtering-Observables.md: review, remove nonexistent operators, fancy like #6131
• Getting-Started.md: fix versions and maven group ids, remove troubleshooting section?, add getting started from Readme.md?
• Home.md: RxJava project is no longer polyglot, link to the other RxYYY projects instead, review "sales pitch", review/expand libraries and check for true v2 version support, mention the v1 interop for the older libs
• How-To-Use-RxJava.md: turn non-Java examples into Java, use only Java examples, fix image links, review the create section
• How-to-Contribute.md: needs extensive rewrite and detailing
• Implementing-Your-Own-Operators.md: needs extensive rewrite, somewhat redundant with Writing-Operators-(2.0).md.
• Implementing-custom-operators-(draft).md: v1 only, unlist it and add banner 1.x content!
• Mathematical-and-Aggregate-Operators.md: review operator list, link to RxJava2Extensions, fancy like #6131, concat is not an aggregator
• Observable-Utility-Operators.md: review set, add new operators, fancy like #6131
• Observable.md: pretty barebone, perhaps list all reactive types and their support interfaces with some descriptions
• Parallel-flows.md: list parallel operators, sequentialDelayError, fancy like #6131, no longer experimental/beta
• Phantom-Operators.md: dumping ground for old/never relaized operators, remove or add warning about historical/museum content?
• Plugins.md: rewrite for RxJavaPlugins and related components
• Problem-Solving-Examples-in-RxJava.md: Fibonacci is not a relevant problem, not sure what problem examples could be listed here instead: how to wrap/bridge to existing technologies? How to get REST data into a listview?
• Reactive-Streams.md: remove v1 related details, wording: Java Flow has been shipped.
• Scheduler.md: needs extensive expansion, also add system properties, perhaps mention exotic schedulers in RxJava2Extensions, perhaps show how to write a scheduler.
• String-Observables.md: the RxJava2Extensions only has a few string operators, link to them?, fancy like #6131
• Subject.md: needs extensive expansion: show the various subject types, their specific methods, examples.
• The-RxJava-Android-Module.md: maybe just link to RxAndroid and remove the rest?
• Transforming-Observables.md: review and add new operators, fancy like #6131
• What's-different-in-2.0.md: fine
• Writing-operators-for-2.0.md: expand: fusion topics, example operators
• _Footer.md: fine
• _Sidebar.md: not sure if this is actually displayed as there is the _Sidebar.md.md too. Sync the two.
• _Sidebar.md.md: reorganize a bit (list categories under "all operators"), remove v1 references, move v2 pages up

This is a broad tracking issue covering many unchecked pages under docs/, including Getting-Started.md, Home.md, and the operator guides. Start by choosing one specific unchecked page and reviewing its current 4.x content against the checklist; done means that page's requested review or update is complete and its checklist item can be marked finished.