Styling for code links in docs is sub-optimal

[GuySartorelli]

Identified during https://github.com/silverstripeltd/product-issues/issues/627
The colour of the underline for code blocks inside links is blue, but the background and text colour for those code blocks is grey. It looks a little odd.
Also, links inside code blocks are not consistent styled with code blocks inside links.

Acceptance Criteria

• code links have a slightly blue-tinted background and blue text
• Appropriate colours are used for code links inside callout blocks e.g. https://docs.silverstripe.org/en/5/changelogs/5.0.0/#trailing-slash
• code a (a link inside a <code> tag) look the same as a code (a <code> tag inside a link)

Notes

• While playing around, Steve and Guy each came up with how we think it should look. Use best judgment or get a designer to opine.
  • Steve's suggestion:

  background-color: #005ae114;
  color: #005AE1;

  • Guy's suggestion:

  background-color: #eef;
  color: #545E6F;

PR

#253