simplifying the 'Symbols' section a bit

[fabi1cazenave]

A quick follow-up after your PR #9.

Wording

As a general statement: when in doubt, I think we should favor a “low profile” wording — concise, humble, to the point. We’re super happy with Arsenik and Selenium, but I believe readers are looking for facts more than enthusiasm. :-)

I’ve proposed a couple changes here to stick a bit more to this principle.

Structure

The more serious modification concerns the structure of the Symbols section, which I see as follows:

• first an explanation of the 3×10 layer itself, which is inherited from Arsenik and Ergol/Lafayette;
• then an explanation of the Selenium-specific thumb cluster trick, which helps typing whole sequences without releasing the Sym key.

And I think the SVG representing the thumb cliusters makes a natural separation between these two parts, without requiring additional headings. Most of this PR is about restoring this structure.

As we seem to share a preference for lists over plaintext, I’ve proposed to factorize the list of left thumb actions so each item can start with a key reference.

WDYT?

[fabi1cazenave]

Before:

After: