Guides: keeping an installation reachable under rate limiting (static homepage + custom 429 page via the reverse proxy)

[ErykKul]

What this PR does / why we need it:

When anonymous traffic (search-engine and AI crawlers) exhausts the shared tier 0 rate-limit bucket, every guest gets 429s — including on the homepage and the login page they'd need to reach a higher tier, so the installation looks down. This adds a field-tested subsection to the Rate Limiting docs with two reverse-proxy recipes that keep the front door usable: serving the homepage as a static file from Apache (zero rate-limit budget, survives app-server saturation) and a styled 429 error page that explains the situation and points guests to login. Includes downloadable sample pages, an nginx pointer, and curl verification steps. This is operational guidance from a production installation, generalized. Docs-only; no linked issue.

[ErykKul]

@pdurbin can you review? A community-contributed subsection for the Rate Limiting docs, based on what we run in production at KU Leuven after bot traffic made the repository look down to guests. @DieuwertjeBloemen pinging you as this documents our setup, feel free to add anything I missed from the operational side.

[DieuwertjeBloemen]

The one thing to definitely include in the documentation is the advice to include a "login" link or button to the login page from both the error page and homepage, as this is the key to properly accessing a Dataverse in these cases. (Without it, people are informed, but can't actually get to the authentication step necessary to by pass the anonymous rate limit).

[ErykKul]

The one thing to definitely include in the documentation is the advice to include a "login" link or button to the login page from both the error page and homepage, as this is the key to properly accessing a Dataverse in these cases. (Without it, people are informed, but can't actually get to the authentication step necessary to by pass the anonymous rate limit).

Good point, made the advice explicit in 1ec2e82.

[Kris-LIBIS]

Is there a need to add that the static landing page is also a good place to add site metadata, like FAIRiCat and have that always available?

[ErykKul]

Is there a need to add that the static landing page is also a good place to add site metadata, like FAIRiCat and have that always available?

Good suggestion, added in 7606244 and refined in 788b956 with the linkset content type and profile.