Update docs and gh-pages workflow to run on `main`

[KoalaGeo]

gh-pages currently runs on for_v1 branch, before going to tagged release v1.0 need to update .yml

There are other changes that we need to the docs, too:

• [x] Update README to use import etlhelper as etl syntax
• [x] Update .rst with latest README changes
• [x] Add new introduction section describing three main components
• [x] Update code section to only list docstrings for "public" modules
• [ ] Add new logo (like the bottom of BGS logo, but with rock layers looking more like table rows)
• [x] Link to v0.14.3 README to retain access to old documentation

[volcan01010]

For @leorudczenko

• [x] Update autogenerated code docs so that they reflect what is available with import etlhelper as etl. This may involve setting __all__ attribute in the __init__.py file.

[leorudczenko]

@volcan01010 I believe I have updated the autogenerated code docs to reflect what is available with import etlhelper as etl and the __init__.py file. I've done this on the overhaul-docs branch.

To do this, I have removed the individual references to each etlhelper module in the index.rst file, and replaced them with a single reference to the etlhelper module as a whole, and an additional reference to etlhelper.row_factories module.

[volcan01010]

This was done in https://github.com/BritishGeologicalSurvey/etlhelper/pull/195/files

We aren't doing the logo for now.