elink, esearch: add page (#8957)

pages/linux/elink.md

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+# elink
+
+> Look up precomputed neighbors within a database, or find associated records in other databases.
+> It is part of the `edirect` package.
+> More information: <https://www.ncbi.nlm.nih.gov/books/NBK179288/>.
+
+- Search pubmed then find related sequences:
+
+`esearch -db pubmed -query "{{selective serotonin reuptake inhibitor}}" | elink -target nuccore`
+
+- Search nucleotide then find related biosamples:
+
+`esearch -db nuccore -query "{{insulin [PROT] AND rodents [ORGN]}}" | elink -target biosample`

pages/linux/esearch.md

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+# esearch
+
+> Perform a new Entrez search using terms in indexed fields.
+> It is part of the `edirect` package.
+> More information: <https://www.ncbi.nlm.nih.gov/books/NBK179288/>.
+
+- Search the pubmed database for selective serotonin reuptake inhibitor:
+
+`esearch -db pubmed -query "{{selective serotonin reuptake inhibitor}}"`
+
+- Search the nucleotide database for sequences whose metadata contain insulin and rodents:
+
+`esearch -db nuccore -query "{{insulin [PROT] AND rodents [ORGN]}}"`