docs: fix links that point at renamed vram headings

[Weijtmans]

The vram.md rewrite (#36, #46) renamed the page's headings, and four inbound links still point at the old ones. mkdocs only reports these at INFO level, so the strict build does not catch them; the links land at the top of the page instead of the intended section.

• linux/alpine.md: #option-1-512mb-dynamic now points at Dynamic VRAM, and the Option 3/Option 4 sentence (both anchors gone) now links What VRAM Split is Best? instead.
• bios/flashing.md: #changing-vram-allocation corrected to #changing-the-vram-split.
• system/sensors.md: the self-link to the atomic-distros subsection had a double hyphen in the slug; the slugifier collapses / and spaces to a single one.

After this, mkdocs build --strict reports zero anchor notes. Since two of the four were orphaned by the vram rewrite I contributed to, this is me cleaning up after it.