initial idea for hierarchical docs

[ashleythomasbarnes]

This pull request adds comprehensive new documentation examples for querying, downloading, and accessing extended metadata from the ESO Science Archive using astroquery. The documentation now covers workflows for APEX data (including Quick Look products), cone search queries, data download options, and retrieval of extended FITS header information.

The most important changes are:

APEX Data Querying and Quick Look Products:

• Added a detailed example notebook (eso_apex.rst) demonstrating how to query for APEX survey data, identify proposal and project IDs, and retrieve APEX Quick Look products, including guidance on the differences between ESO programme IDs and APEX proposal IDs.

Cone Search Queries:

• Introduced a new example (eso_cone_search.rst) showing how to perform cone searches for both reduced and raw data products using various astroquery methods, with step-by-step instructions and code for querying around specific sky coordinates.

Data Download Workflows:

• Added documentation (eso_download.rst) explaining how to download datasets by data product ID, customize the download destination, force data re-download, and optionally include associated calibration files (raw or processed).

Extended FITS Header Retrieval:

• Provided an example (eso_header_info.rst) for retrieving the full primary FITS header for data products using get_headers, illustrating how to access all available metadata columns for each dataset.