Add docs/FAQ for breaking changes to markup

[TomJGooding]

This is obviously a contrived example, but imagine you have created an app something like this:

from textual.app import App, ComposeResult
from textual.widgets import Button, Static, Tabs


class ExampleApp(App):
    BINDINGS = [
        ("esc", "cancel"),
        ("1", "switch_tab('tab-1')"),
        # etc...
    ]

    def compose(self) -> ComposeResult:
        yield Tabs("First [1]", "Second [2]", "Third [3]")
        yield Static(
            '[[1], [1,2,3,4], ["hello"], [None], [False], [True]]',
        )
        yield Button("Cancel [Esc]", variant="error")


if __name__ == "__main__":
    app = ExampleApp()
    app.run()

Before Textual v2.0.0, it doesn't matter that you haven't escaped any of the square brackets:

But now in the latest version of Textual (v3.0.0) the tabs and static widget will crash with a MarkupError. Apparently the button doesn't crash, but instead just removes the [Esc] text. I'm not sure what causes some invalid tags to crash while others just removed?

I suspect this will be a common issue, not only for users upgrading from a previous version but also developers new to Textual. I think it is worthwhile adding something about handling invalid markup to the docs and/or FAQ.