MRG: revise documentation structure; add internals page.

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+# Welcome to sourmash!
+
+sourmash is a command-line tool and Python/Rust library for
+**metagenome analysis** and **genome comparison** with k-mers.  It
+supports the compositional analysis of metagenomes, rapid search of
+large sequence databases, and flexible taxonomic analysis with both
+NCBI and GTDB taxonomies. sourmash works well with sequences 30kb or
+larger, including bacterial and viral genomes.
+
+You might try sourmash if you want to -
+
+* identify which reference genomes are present in a metagenome
+* search all Genbank microbial genomes with a genome query
+* build an annotation-free clustering of many genomes using k-mers or average nucleotide identity (ANI)
+* taxonomically classify genomes or metagenomes against NCBI and/or GTDB;
+* search all available reference genomes
+* search thousands of metagenomes with a query genome or sequence
+
+Underneath, sourmash uses [FracMinHash sketches](@@) for fast and
+lightweight sequence comparison; FracMinHash builds on
+[MinHash sketching](@@wikipedia) to support both Jaccard similarity
+_and_ containment analyses with k-mers.  This significantly expands
+the range of operations that can be done quickly and in low
+memory. sourmash also implements a number of new and powerful analysis
+techniques, including minimum metagenome covers and alignment-free ANI
+estimation.
+
+sourmash is inspired by [mash](https://mash.readthedocs.io), and
+supports most mash analyses. sourmash also implements an expanded set
+of functionality for metagenome and taxonomic analysis.
+
+## Using sourmash
+
+### Tutorials and examples
+
+These tutorials are command line tutorials that should work on Mac OS
+X and Linux. They require about 5 GB of disk space and 5 GB of RAM.
+
+* [The first sourmash tutorial - making signatures, comparing, and searching](tutorial-basic.md)
+
+* [Using sourmash LCA to do taxonomic classification](tutorials-lca.md)
+
+* [Analyzing the genomic and taxonomic composition of an environmental genome using GTDB and sample-specific MAGs with sourmash](tutorial-lemonade.md)
+
+* [Some sourmash command line examples!](sourmash-examples.md)
+
+### How-To Guides
+
+* Installing sourmash
+
+* [Classifying genome sketches](classifying-signatures.md)
+
+* [Working with private collections of genome sketches.](sourmash-collections.md)
+
+* [Using the `LCA_Database` API.](using-LCA-database-API.ipynb)
+
+* [Building plots from `sourmash compare` output](plotting-compare.md).
+
+* [A short guide to using sourmash output with R](other-languages.md).
+
+### How sourmash works under the hood
+
+* [An introduction to k-mers for genome comparison and analysis](kmers-and-minhash.md)
+* [Support, versioning, and migration between versions](support.md)
+
+### Reference material
+
+* [UNIX command-line documentation](command-line.md)
+* [Genbank and GTDB databases and taxonomy files](databases.md)
+* [Python examples using the API](api-example.md)
+* [Publications about sourmash](publications.md)
+* [A guide to the internals of sourmash](sourmash-internals.md)
+
+## Developing and extending sourmash
+
+* [Releasing a new version of sourmash](release.md)