Move legacy APM books back to /en/apm/*

[bmorelli25]

Amends #2309 by moving legacy APM books back to /en/apm/*.

[gtback]

We can see what this looks like, but I was thinking it might be easier to just manually add en/apm prefix on lines 2300/2303 and 2316/2319. As it is, this is likely to break things, since the page https://www.elastic.co/guide/en/apm/index.html can't come from two different sections.

[gtback]

Or, honestly, we could move the books back up into the APM section (on that same page linked above), but have a subsection on that page (maybe?)

[bmorelli25]

Or, honestly, we could move the books back up into the APM section (on that same page linked above), but have a subsection on that page (maybe?)

That's a reaaaaaally interesting idea. A legacy section within that subsection would probably make more sense for our users anyways since they're more likely to look in the APM section for these docs.

[gtback]

Looks good, assuming the diff shows what we expect. We can clean up the "new" "legacy" docs at the wrong URL in the built-docs repo once this gets merged.