ESQL: Standardize font used in railroad diagrams (#98897)

Locks the railroad diagrams to always use the same font, this one named
`roboto mono`. This makes sure that when we render the railroad diagrams
we always size them the same way. Because everyone has a copy of roboto
mono. Because gradle resolves that dependency.

Author: nik9000

docs/reference/esql/functions/abs.asciidoc

@@ -1,7 +1,7 @@
 [[esql-abs]]
 === `ABS`
 [.text-center]
-image::signature/abs.svg[]
+image::esql/functions/signature/abs.svg[Embedded,opts=inline]
 
 Returns the absolute value.
 

docs/reference/esql/functions/cosh.asciidoc

@@ -1,7 +1,7 @@
 [[esql-cosh]]
 === `COSH`
 [.text-center]
-image::signature/cosh.svg[]
+image::esql/functions/signature/cosh.svg[Embedded,opts=inline]
 
 https://en.wikipedia.org/wiki/Hyperbolic_functions[Cosine] hyperbolic function.
 

docs/reference/esql/functions/greatest.asciidoc

@@ -1,7 +1,7 @@
 [[esql-greatest]]
 === `GREATEST`
 [.text-center]
-image::signature/greatest.svg[]
+image::esql/functions/signature/greatest.svg[Embedded,opts=inline]
 
 Returns the maximum value from many columns. This is similar to <<esql-mv_max>>
 except it's intended to run on multiple columns at once.

docs/reference/esql/functions/pow.asciidoc

@@ -1,7 +1,7 @@
 [[esql-pow]]
 === `POW`
 [.text-center]
-image::signature/pow.svg[]
+image::esql/functions/signature/pow.svg[Embedded,opts=inline]
 
 Returns the value of a base (first argument) raised to the power of an exponent (second argument).
 Both arguments must be numeric.