Improve documentation of int

[MewPurPur]

Main description:

• Explained float->int and int->float conversions in the description.
• Added a note about int in boolean context.
• Explained GDScript's separators _ and binary/hex literals 0b and 0x.

The rest of the page:

• Followed how other classes are documented more closely.
• Shortened the line count of examples dramatically, using one good example to demonstrate each basic concept.
• Fixed typos and misteaks.
• Used 0b to explain bitwise operators.
• Misc.

Changed how operator descriptions are worded:

• int + float -> "the int", "the float"
• int + int -> "the two ints" / "the left int", "the right int"

[aaronfranke]

Since you removed the mention of type hints from the above text, we should probably mention it in the code sample. Particularly since var x = 1 would also work and be an integer, the difference is that it would change the behavior of x = 4.2, with the hint it's casted to 4, without it stores the float 4.2. We want to inform users that : int is not a required part of using the integer type.

[MewPurPur]

Hmm, I don't know how to do this gracefully. While we wouldn't want to explain how static typing works here, we do want to explain the whole back-and-forth between int and float (for example how passing an int to a function that accepts a float will just do an appropriate conversion). Maybe I could explain that this also holds for statically-typed vars as an example?

[YuriSizov]

This is great, thanks!

[YuriSizov]

Cherry-picked for 4.0.2.