Refactor: Sync Contributing section from Scala-lang Guide to Dotty website (#17459)

Author: Dedelweiss

docs/_assets/images/contribution/breakpoint.jpg

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+---
+layout: doc-page
+title: Contexts
+---
+
+`dotc` has almost no global state (with the exception of the name table,
+which hashes strings into unique names). Instead, all
+essential bits of information that can vary over a compiler [run](./lifecycle.md) are collected
+in a `Context` (defined in [Contexts]).
+
+Most methods in the compiler depend on an implicit anonymous `Context` parameter,
+and a typical definition looks like the following:
+```scala
+import dotty.tools.dotc.Contexts.{Context, ctx}
+
+def doFoo(using Context): Unit =
+  val current = ctx.run // access the Context parameter with `ctx`
+```
+
+## Memory Leaks
+> **Careful:** Contexts can be heavy so beware of memory leaks
+
+It is good practice to ensure that implicit contexts are not
+captured in closures or other long-lived objects, in order to avoid space leaks
+in the case where a closure can survive several compiler runs (e.g. a
+lazy completer for a library class that is never required). In that case, the
+convention is that the `Context` be an explicit parameter, to track its usage.
+
+## Context Properties
+
+| Context property  | description                            |
+|-------------------|----------------------------------------|
+| `compilationUnit` | current compilation unit               |
+| `phase`           | current phase                          |
+| `run`             | current run                            |
+| `period`          | current period                         |
+| `settings`        | the config passed to the compiler      |
+| `reporter`        | operations for logging errors/warnings |
+| `definitions`     | the standard built in definitions      |
+| `platform`        | operations for the underlying platform |
+| `tree`            | current tree                           |
+| `scope`           | current scope                          |
+| `typer`           | current typer                          |
+| `owner`           | current owner symbol                   |
+| `outer`           | outer Context                          |
+| `mode`            | type checking mode                     |
+| `typerState`      |                                        |
+| `searchHistory`   |                                        |
+| `implicits`       |                                        |
+| ...               | and so on                              |
+
+
+[Contexts]: https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/core/Contexts.scala