docs(readme): name the Rx-compatible alias for every factory

[NeverMorewd]

What kind of change does this PR introduce?

Docs update.

What is the new behavior?

The factory mapping names the Rx-compatible alias alongside the Primitives name, for every row that has one.

• Observable.Interval(period) lists Signal.Pulse, Signal.Every and Signal.Interval.
• The same treatment for Return, Empty, Never, Throw, Range, Repeat, Defer, Using and Timer, each of which also ships under its Rx name.
• Seven factories that had no row at all are documented: Generate, If, Case, Concat, Merge, Switch and OnErrorResumeNext.

What is the current behavior?

The table gave only the Primitives name for most rows, so a reader migrating from System.Reactive could not tell from it that the Rx name resolves too, and the factories above were absent.

What might this PR break?

None. Documentation only.

Checklist

• I have read the Contribute guide
• Tests have been added or updated (for bug fixes / features)
• Docs have been added or updated (for bug fixes / features)
• Changes target the main branch
• PR title follows Conventional Commits

Additional information

Every alias named here was checked against the Signal{RxAliases} sources, so each one exists with the signature shown.

[glennawatson]

@NeverMorewd apologies - some unrelated commits landed on your branch and left it conflicting. They have been moved off, so this is back to your README change, rebased and green.

You were right that Signal.Interval was missing, so I extended the table to name the Rx alias on every row that has one. You are co-author on that commit. Nothing needed from you - thanks for the fix.

[glennawatson]

@ChrisPulman five of your commits landed on this contributor's fork branch rather than your own - looks like an agent running without sandboxing. Nothing lost: they are on CP_sequencer-delay-seam at 0f457178, ready to raise separately.