In signatures.rakudoc: New material on type captures; work on introduction and parameter separators; some fixes here and there

[schultzdavid]

(The first three commits have already been merged to Raku:main via my pull request 4648 last week.)

This new bunch does a couple of things:

• In the section Type captures, I added an example that combines type captures with type coercion (taken from roast github.com/Raku/roast/blob/master/S06-signature/type-capture.t), and I further examples combining that with type constraints (conformant with the 4th example in this section, although strictly speaking I don't know if the examples are supposed to work as they do (there seem to be no roast for type constraints on captures)).
• In the introductory section, to keep it about only the most relevant things, I moved the part about the invocant marker : to the next section ("Parameter separators") by merging the two very similar examples into one.
  • Still to do: In a future commit (tomorrow probably) I will add a part about slurpy parameters in the intro section.
• I fixed some links, e.g. one link was broken because "Typesystem" was erroneously capitalized.
• I shuffled some words and sentences around, although that's probably debatable

[schultzdavid]

These last two commits fix a sentence relationship that got a bit out of order through the addition of material in commit c286df1. Also, the examples should now be easier to digest as they each illustrate one new point at a time.

[schultzdavid]

About the last 4 commits and the next one: That one can use type captures as return types, is indeed intended (e.g. S06-signature/type-capture.t#L98).

[schultzdavid]

These last two commits (adding text on slurpies to the introduction) are now really the last ones to this PR.