Add rgbasm-old.5 as an "Obsolete syntax" page

[Rangi42]

This is meant to go along with gbdev/rgbds#1571, so that when RGBDS 0.9.0 releases this month, it will have an online document of obsolete unsupported instructions and directives.

The main problem I'm running into is that previous versions of RGBDS did not have this new page, so yarn build fails. Maybe we should manually create stub rgbasm-old.5 pages for the old versions?

Feel free to push more commits to this branch to get it to work.

[avivace]

I'm taking a look at this. A bit strange that yarn start lets you serve it with the dev server but build fails ..

[avivace]

@Rangi42 so, it seems like the error is correctly thrown, because we are redirecting /docs/<page> links to docs/<latestStable>/<page> ones. Since that page doesn't exist for 0.8.0 it can't create the placeholder page making the redirection.
I'd say we can start merging this without this specific redirection (so the page only appears in master version), the add it back when we release 0.9.0 and the redirect will make sense.

[avivace]

LGTM. I'll open an issue for new redirection.

[Rangi42]

Thanks! With that change, yarn build works, and when I serve with yarn start, the "Obsolete syntax" sidebar item appears only in the master version, not the previous ones. Which is as expected.