[lldb/infra] Revamp lldb.llvm.org

This patch brings a fresh coat of paint to the LLDB website.

This uses the `furo` theme from the suggested theme list, adds some
changes to the css to make it use the full browser window width and
removes the table of contents since the theme generates it automatically.

This patch also rewrites the tables for "GDB to LLDB command map" to
convert them from raw HTML to native inlined code blocks.

To generate the website, make sure you install the theme first with
`pip install furo`, enable sphinx in the cmake invocation` then run
`ninja docs-lldb-html`

Discourse: https://discourse.llvm.org/t/rfc-revamping-lldbs-website

Differential Revision: https://reviews.llvm.org/D158323



Signed-off-by: Med Ismail Bennani <ismail@bennani.ma>