[X3] Deprecated features and suggested solutions should be documented

[saki7]

As part of C++23 modernization (#809), I've deprecated many features. Such deprecation falls into below categories:

• Error-prone API (including potential security issues)
  • e.g. operator/ for semantic actions
• Semantically wrong API
  • e.g. any_parser
• Old interface that uses obsolete Boost features
  • e.g. boost::optional

We have these types of "deprecation" in practice:

• Entities marked as [[deprecated]].
• Entities immediately removed for various technical reasons, including security concerns.
• Functionalities that has the old and the new API, which are switchable by certain macros.

We'd better list such deprecations on README.md and document the suggested solutions.

The information would be the good starting point ("you should define these macros by default" kind of guideline) for new applications.