Github.io changelog: increase the font-weight of the level 1 heading

[Basilisk3]

Description of the proposed changes

As per the title. Currently, h2 has a higher weight than h1, which looks a bit odd.

[lL1l1]

Tested by modifying using inspect element.
Before:

After:

[Basilisk3]

Looks like this change is overridden in base.scss.

Replacing the code added in this PR with the code below could fix this, but since it is not possible to test changes to the .scss via inspect element, I cannot be sure.

.custom-container h1,
.custom-container .text-alpha {
  font-weight: 600;
}