Activate portable feature in mithril-stm by default

[jpraynaud]

Why

blst library has implemented a runtime switch to ADX with the portable feature since 0.3.11. We want to build our binaries in the CI with this feature to avoid having some users forced to compile the binaries on their machine when their CPU is not compatible with ADX.

What

Compile pre-built binaries in CI with portable feature. If possible, we can get rid of the portable feature.

How

• Compile pre-built binaires with portable feature
• Activate portable as default in our crates
• Get rid of portable feature if possible and simplify CI workflow
• Remove mention to manual build in documentation SPO guide (see PR)