Rudimentary .gitignore suitable for docs

[cameel]

While converting the repository for the translation bot today, I noticed that the repository contained a lot of files that are not suitable for git versioning. I removed them but to ensure that they won't be committed again, the repository should have a .gitignore file. This file tells git to hide these files when making a commit. This PR adds such a file.

[cameel]

Content removed during the conversion

By the way, this is what I removed from some older commits during the conversion:

• docs.zip. Files in the repository should be stored ucompressed.
• solidity0.8.4.pdf and solidity_translate_pdf.
• Built documentation under docs/_build/. These files are generated automatically from the .rst files and should not be committed.
• docs/_static/robots.txt. This file is also created automatically when you build the docs.
• __pycache__/ directories. These get created during compilation of Python code - the docs contain two small custom extensions written in Python.

Some of them already had commits deleting them from the current revision but this does not remove them from older revisions and they still take up space. Since I had to rewrite history anyway, I took the opportunity to remove them completely.

I have also deleted the old master branch, which had history completely disconnected from main and contained only very short index.html and style.css files. They haven't been touched in a long time and did not look like something that needs to be preserved.

In case you want to go back to any removed files for any reason, you can still access the original main branch, though I recommend deleting it if you have no issues with the end result of the conversion.