packaging_guide.rst: explain forward and backward compat before the less common cases (#47402)

The idea is to go from most to least used: backward compat -> forward compat -> pinning on major or major.minor version -> pinning specific, concrete versions.

Further, the following

```python
   # backward compatibility with Python
   depends_on("python@3.8:")
   depends_on("python@3.9:", when="@1.2:")
   depends_on("python@3.10:", when="@1.4:")

   # forward compatibility with Python
   depends_on("python@:3.12", when="@:1.10")
   depends_on("python@:3.13", when="@:1.12")
   depends_on("python@:3.14")
```

is better than disjoint when ranges causing repetition of the rules on dependencies, and requiring frequent editing of existing lines after new releases are done:

```python
   depends_on("python@3.8:3.12", when="@:1.1")
   depends_on("python@3.9:3.12", when="@1.2:1.3")
   depends_on("python@3.10:3.12", when="@1.4:1.10")
   depends_on("python@3.10:3.13", when="@1.11:1.12")
   depends_on("python@3.10:3.14", when="@1.13:")

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